Belt device and unit device including belt device and image forming apparatus using the belt device and unit device

An image forming apparatus suppresses several kinds of inconveniences caused by unnecessary contact of a belt-formed member with opposing members and drives the belt-formed member accurately even when the belt-formed member separated from a part of a plurality of opposing members. In an image forming apparatus having a belt-formed member supported by a plurality of supporting rollers and a plurality of opposing members located side by side in a line to oppose and contact the belt-formed member, a pivot mechanism is employed to temporarily separate the belt-formed member from a part of the opposing members for color image formation. The image forming apparatus also includes a tension roller dive mechanism to increase a relative distance between the tension roller and other supporting rollers to suppress a decrease in a tension of the belt-formed member during the above-described separation of the belt-formed member from the plurality of opposing members.

CROSS-REFERENCE TO RELATED APPLICATIONS

This document claims priority and contains subject matter related to Japanese Patent Applications Nos. JPAP11-166288 filed on Jun. 14, 1999, JPAP11-365318 filed on Dec. 22, 1999 and JPAP2000-114451 filed on Apr. 14, 2000, and the entire contents thereof are herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus such as, a copying machine, a facsimile, a printer, etc., and more particularly to an image forming unit device including a belt-formed member and a belt device in which the belt-formed member drives accurately even when the belt-formed member temporarily separates from some of opposing members.

2. Discussion of the Background

As an image forming apparatus, a tandem multicolor image forming apparatus, that includes an intermediate transfer element supported by a plurality of supporting rollers and a plurality of photoconductive elements as opposing members (image bearing members) arranged side by side in a line opposite to the intermediate transfer element and contacting the intermediate transfer element is known (e.g. in Japanese Utility Model Laid-Open No. 59-192159 and Japanese Patent Laid-Open publication No. 8-160839). In the apparatus, visible images corresponding to respective colors formed on surfaces of respective photoconductive elements are transferred onto the intermediate transfer element one after another while being superimposed with each other (a primary transfer). The visible image thus formed on the intermediate transfer element is then transferred onto a transfer member at one time (a secondary transfer) to form a multicolor image on the transfer member. In those multicolor image forming apparatuses, there are apparatuses configured such that a black and white image forming mode using a single photoconductive element and a multicolor image forming mode superimposing toner images of a plurality of colors with each other using a plurality of photoconductive elements are selectable.

FIG. 27illustrates a fullcolor electrophotographic copying machine using liquid developer as an example of the above-described tandem multicolor image forming apparatus. In the apparatus, four drum-shaped photoconductive elements501Y,501M,501C and501B corresponding to respective colors of yellow Y, magenta M, cyan C and black BK are provided side by side in a line such that the axes of rotation of photoconductive elements are located in parallel and in the same plane. Around respective photoconductive elements501Y,501M,501C and501B rotating in a clockwise direction, charging devices, writing systems to form an electrostatic image by irradiation of beam light corresponding to respective colors, developing units with liquid developer for respective colors etc. (not shown) are provided respectively in an order of a liquid electrophotographic printing process. Further, an intermediate transfer belt505as an intermediate transfer member is supported by a tension roller502, guide rollers503and504etc. so as to rotate in a counterclockwise direction. The intermediate transfer belt505is disposed so as to contact each primary transfer area of photoconductive elements501Y,501M,501C and501B. The intermediate transfer belt505is pressed by spanning rollers506Y,506M,506C and506B so that it windingly contacts respective photoconductive elements. An image on the intermediate transfer belt505, which has been formed as a result of transferring images of respective colors (Y, M, C and BK) at the primary transfer areas of respective photoconductive elements501Y,501M,501C and501B superimposing one after another, is conveyed to a secondary transfer area where a portion of the intermediate transfer belt505spanned between guide rollers503and504contacts a secondary transfer roller507. Then, the image is transferred onto a transfer sheet508at the secondary transfer area to form a multicolor image on the transfer sheet508. Further, a cleaning device509is provided at a place where the intermediate transfer belt505is supported by the tension roller502.

In the fullcolor electrophotographic copying machine with liquid developer, a color mode can be freely selected from among, for example, a single color mode and a multicolor mode with four colors (a full color mode), two colors or three colors. For example, when a single color mode (black color mode) is selected, a black color image is formed on the transfer sheet508using the photoconductive element501B, electrophotographic copying process members and the intermediate transfer belt505.

When a single color image forming operation is performed in the above-described electrophtographic copying machine having selectable single color and multicolor modes, inconveniences may be caused because the photoconductive elements which are not involved in the image forming operation are located in contact with or in close proximity to the intermediate transfer element.

For example, life times of the photoconductive elements may be decreased because the photoconductive elements are kept in contact with the intermediate transfer element even when the photoconductive elements are not involved in the image forming operation. In the apparatus illustrated inFIG. 27, even in the black color mode, photoconductive elements501Y,501M and501C, which are not involved in the image forming operation, are kept in contact with the intermediate transfer belt505and are rubbed by it. Therefore the life times of these photoconductive elements may be decreased.

Further, when photoconductive elements which are not involved in the image forming operation are kept in contact with or in close proximity to the intermediate transfer element, developer remaining on the photoconductive elements may be flown by the intermediate transfer element and scattered inside the apparatus. Developer remaining on the photoconductive elements may also adhere to a surface of the intermediate transfer element, which results in unnecessary consumption of developer.

The above-described inconveniences such as the life times of opposing members, such as photoconductive elements being decreased due to unnecessary contact of a belt-formed member, such as the intermediate transfer element, with the opposing members are caused not only in the above-described exemplary construction where a plurality of photoconductive elements are located side by side in a line so as to oppose and contact the belt-formed intermediate transfer element, but also in a construction where a plurality of opposing members are disposed side by side in a line so as to oppose and contact a belt-formed member supported by a plurality of supporting rollers driven while being temporarily separated from part of the plurality of opposing members. The above-described inconveniences are also caused, for example, in a construction where a belt-formed photoconductive element drives while the belt-formed photoconductive element is temporarily separated from part of a plurality of developer bearing members as the plurality of opposing members, or in a construction where a belt-formed transfer sheet conveying member drives while the belt-formed transfer sheet conveying member is temporarily separated from part of a plurality of photoconductive elements as the plurality of opposing members. Further, the above-described scattering of developer and unnecessary consumption of the developer occur not only when the plurality of opposing members are located side by side in a line opposing and contacting the belt-formed member but also when the plurality of opposing members are located side by side in a line opposing the belt-formed member in close proximity.

For example, in Japanese Patent Laid-Open Publication No. 9-146383, an example of an image forming apparatus, configured such that a transfer sheet conveying belt partly moves to separate from three photoconductive elements out of four, is described.

The inventors discovered the following shortcoming as a result of a further study on a construction that enables the intermediate transfer element as the belt-formed member to separate from part of the plurality of photoconductive elements as the plurality of opposing members. When the intermediate transfer element is separated from part of the photoconductive elements that are not involved in the image forming operation, a tension of the intermediate transfer element may vary. For example, when the intermediate transfer element is configured to contact each of the photoconductive elements with a certain contacting angle in order to form a primary transfer nip of a required width between the intermediate transfer element and each photoconductive element, the tension of the intermediate transfer element may be decreased when the intermediate transfer element is separated from some of the photoconductive elements which are not in use. Further, when part of a plurality of supporting rollers pivot in order to separate the intermediate transfer element from part of the photoconductive elements which are not involved in the image forming operation, the tension of the intermediate transfer element may be decreased or increased depending on a position of a pivot.

When the intermediate transfer element is driven while the tension has varied, the intermediate transfer element may not be driven accurately. For example, when the intermediate transfer element is frictionally driven by rubber rollers, if the tension of the intermediate transfer element is decreased, the intermediate transfer element may not be accurately driven by the rubber rollers due to slides of the intermediate transfer element over the rubber rollers. Contrarily, if its tension is increased, a driving load imposed on the intermediate transfer element may become too excessive to drive the intermediate transfer element accurately. What is meant herein by saying that the intermediate transfer belt is driven accurately is to minimize a change in the speed of the intermediate transfer element.

The above-described inconvenience of inaccurate drive of a belt-formed intermediate transfer element due to a variation in the tension of the intermediate transfer element may be caused not only when a plurality of photoconductive elements are disposed side by side in a line opposing and contacting the belt-formed intermediate transfer element as described above, but also when a plurality of opposing members are arranged side by side in a line opposing and contacting or in close proximity to a belt-formed member supported by a plurality of supporting rollers frictionally driven while being temporarily separated from part of the plurality of opposing members. For example, the inconvenience may also be caused when a belt-formed photoconductive element is driven while being separated from part of a plurality of developer bearing members as a plurality of opposing members or when a belt-formed transfer sheet conveying member is driven while being temporarily separated from part of a plurality of photoconductive elements as a plurality of opposing members. Further, the inconvenience may also be caused not only when the plurality of opposing members are arranged side by side in a line so as to contact the belt-formed member but also when they are arranged side by side in a line so as to oppose the belt-formed member in close proximity.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above-discussed and other problems and addresses the above-discussed and other problems.

The present invention advantageously provides a novel image forming apparatus, an image forming unit device having a belt-formed member and a belt device for use in the image forming apparatus, for preventing inconveniences caused by unnecessary contact of the belt-formed member with opposing members, or proximity of the two members by making it possible to separate the belt-formed member from part of the opposing members.

The present invention also advantageously provides a novel image forming apparatus, an image forming unit device having a belt-formed member and a belt device for use in the image forming apparatus, for driving the belt-formed member accurately even when the belt-formed member is separated from part of a plurality of opposing members located in close proximity to the belt-formed member or contacting the belt-formed member.

According to an embodiment of the present invention, an image forming apparatus includes a belt-formed member supported by a plurality of supporting rollers, the belt-formed member being a belt-formed intermediate transfer element, a plurality of opposing members located side by side in a line and opposing said belt-formed member, each of the plurality of opposing members being a latent image bearing member to form a latent image to be transferred onto the intermediate transfer element and a separation device to separate the intermediate transfer element located in close proximity to the plurality of latent image bearing members or in contact with the plurality of latent image bearing members from part of the plurality of latent image bearing members.

Other objects, features and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views,FIG. 1is a schematic drawing illustrating an internal construction of an electrographic multicolor printer with liquid developer (hereinafter referred to as printer) as an example of an image forming apparatus according to an embodiment of the present invention. The printer receives image data from a personal computer (PC) etc., and performs a printing process.

As illustrated inFIG. 1, four drum-shaped photoconductive elements10Y,10M,10C and10B, as opposing members (latent image bearing members), corresponding to respective colors of yellow Y, magenta M, cyan C and black B, are disposed side by side in a line. Each axis of rotation of the photoconductive elements10Y,10M10C and10B is located in the same plane and in parallel with each other axis. The photoconductive element10B for a black color mode (single color mode) is located close to a common secondary transfer area.

Above the photoconductive elements10Y,10M,10C and10B, an intermediate transfer unit70is removably provided to a main body of the apparatus. The intermediate transfer unit70includes an intermediate transfer belt100in an endless form as a belt-formed member (an intermediate transfer element) supported by a plurality of rotatable supporting rollers70-76and80. The intermediate transfer belt100is spanned around spanning roller74-76and80, as supporting rollers so as to windingly contact part of respective photoconductive elements10Y,10M,10C and10B.

Primary transfer rollers (not shown) are located at positions opposite to respective photoconductive elements interposing the intermediate transfer belt100between those primary transfer rollers and a respective photoconductive element. A transfer bias may be applied to the primary transfer roller as necessary. In the secondary transfer area, where a toner image is transferred from the intermediate transfer belt100onto a transfer sheet200, located along a sheet conveying path for the transfer sheet, a secondary transfer roller81is provided press-contacting the intermediate transfer belt100and spanned around a driving roller72and a guide roller73as supporting rollers. A transfer bias may also be applied to the secondary transfer roller81as necessary.

For the intermediate transfer belt100, a belt configured to be a double layer structure may be used. A first layer including an elastic member formed on a surface side where toner image is formed and a second layer including a resin sheet on back side thereof and having a volume resistivity of 107to 1012Ωcm may be used. For the first layer, a polyurethan rubber layer of 200 to 700 μm in thickness may be used, and as for the resin sheet layer, a polyurethan resin sheet of 100 to 500 μm in thickness and which is not stretched in a circumferential direction may be used. Further, the intermediate transfer belt100may include a combination of a first layer of rubber on the surface (e.g. a nitrile rubber, a urethan rubber, the Butyl-rubber and a natural rubber) and a second layer of a fiber buried rubber, or a combination of the first coated layer including a fluorine resin and the second layer of an elastic conductive element having a volume resistivity of 105to 109Ωcm, where a non-elastic core (e.g. a nylon cord and a steel cord) is extendedly buried in the circumferential direction.

For supporting rollers71-76and80, a grounded conductive roller (e.g. a metal roller) may be used. As for the primary transfer roller77and the secondary transfer roller81, a columned or cylindrical-shaped conductive roller having a conductive rubber layer on its surface (e.g. a metal roller or a metal pipe) may be used. When the intermediate transfer belt100having a conductive layer on its underside is used, a floating state conductive roller (e.g. a metal roller) or a nonconductive roller is used for supporting rollers72-76and80other than the tension roller71and the primary transfer roller.

The tension roller71is made of a conductive roller so that the conductive layer of the intermediate transfer belt100has a predetermined potential by a bias voltage applied to the tension roller71. When the transfer bias is applied to the secondary transfer roller81, a transfer electric field is formed by the potential difference between the conductive layer of the intermediate transfer belt100and the secondary transfer roller81. Around the respective photoconductive elements10Y,10M,10C and10B, electrophotographic image forming processing members, such as charging devices20Y,20M,20C and20B and developing units with liquid developer40Y,40M,40C and40B are provided in order of the image forming process. Further, light irradiating paths where laser beam light is irradiated through are also disposed around respective photoconductive elements10Y,10M,10C and10B. Because developing units with liquid developer40Y,40M,40C and40B have the sane structure as to each other except containing toners of different colors, those developing units can be replaced with respect to each other.

A sheet transfer path202is formed to convey the transfer sheet200from a sheet feeding tray201located below photoconductive elements10Y,10M,10C and10B to the secondary transfer area. A registration roller203to adjust a time to feed the transfer sheet200is located right before, in a sheet conveying direction, a guide roller73which is one of the supporting rollers. A first conveying belt unit204, a primary fixing unit91, a secondary conveying belt unit205, a secondary fixing unit92, an exit tray206, etc., are properly located along a sheet discharging path207at a downstream side of the secondary transfer area with respect to the transfer sheet conveying direction.

In the printer according to the embodiment of the present invention, the tension roller71and spanning rollers75,76and80are pivoted about a shaft of the driving roller72so as to be vertically swingable. By the pivotal movement of the tension roller71and spanning rollers75,76and80, part of the intermediate transfer belt100, which is an intermediate transfer element (a belt-formed member), pivots around the shaft of the driving roller72to vertically move. As a result, the intermediate transfer belt100can be positioned either at a place where the intermediate transfer belt100contacts all of the photoconductive elements10Y,10M,10C and10B or a separated position where the intermediate transfer belt100contacts only the photoconductive element10B, separated from other photoconductive elements10Y,10M and10C. The separation of the intermediate transfer belt100from part of photoconductive elements10Y,10M and10C is achieved by a belt position change mechanism110that changes the positions of the tension roller71and spanning rollers75,76and80through a belt uplift mechanism111U and a belt lift down mechanism111D illustrated in FIG.3.

According to the embodiment of the present invention, a cleaning device79to clean the intermediate transfer belt100is located at the side of the pivot of the intermediate transfer belt100instead of a position where the cleaning device509is placed in FIG.27. In other words, the cleaning device79is provided at a position opposed to the driving roller72which is the center of the pivot. Though a blade-formed cleaning device is illustrated inFIG. 1as an example of the cleaning device79, the cleaning device79may be formed like a roller, web or the like.

FIG. 3is a block diagram explaining a data process control system of the printer according to the embodiment of the present invention. A decoder120receives image data transmitted from a personal computer (PC), converts it to image data corresponding to respective colors and then bit-maps each image data so as to be stored in page memories121Y,121M,121C and121K. A mode determination circuit122determines between a single color mode (black color mode) and a multicolor mode such as a full color mode based on the received image data. An engine control CPU (central processing unit)123, which functions as a drive control device and a control device to control operations of each unit of the printer, is connected to the mode determination circuit122.

When the mode determination circuit122recognizes the multicolor mode for a full color based on the image data transmitted from the personal computer PC, the engine control CPU123activates the belt lift down mechanism111D. Then the belt position change mechanism110lifts down the tension roller71etc. to a position indicated by a solid line inFIG. 2so as to contact the primary transfer areas of the photoconductive elements10Y,10M,10C and10B, which is an initial position of the intermediate transfer belt100(hereinafter a returning of the intermediate transfer belt to the initial position is referred to as replacement of the intermediate transfer belt). A multicolor image formation by superimposing respective color toner images on each other becomes possible by the replacement of the intermediate transfer belt100. The replacement of the intermediate transfer belt100is performed while image data for the multicolor image formation is being bit-mapped and stored in respective page memories121Y,121M,121C and121B (four times longer than a time for a single color). Therefore, the apparatus can be set ready for a multicolor image forming operation without requiring an additional time for the process. Similarly, the intermediate transfer belt100can be cleaned several times by the cleaning device79by rotating the intermediate transfer belt100while image data for the multicolor image formation is being bit-mapped and stored in respective page memories121Y,121M,121C and121B, and thereby a cleanliness of the intermediate transfer belt100is increased without taking an additional time for the cleaning.

Contrarily, when the mode determination circuit122recognizes the single color mode based on the image data transmitted from the personal computer PC, the engine control CPU123activates the belt uplift mechanism111U so that the belt position change mechanism110swingingly moves the tension roller71and spanning rollers75,76and80etc. to a separated position indicated by a dotted line inFIG. 2, where the intermediate transfer belt100contacts only the photoconductive element10B and is separated from other photoconductive elements10Y,10M and10C. As a result, an operation for an image forming and printing of the black color mode with the photoconductive element10B, surrounding developing unit40B with liquid developer, the intermediate transfer belt100and so forth becomes possible. Consequently, although the intermediate transfer belt100rotates as in a case of the multicolor mode, the intermediate transfer belt100does not contact photoconductive elements10Y,10M and10C which are not involved in the image formation and printing process, and thereby the life of photoconductive elements10Y,10M and10C may not be decreased. Especially, because the black color mode, which is most frequently used, is set as the single color mode, the life of photoconductive elements10Y,10M and10C may be advantageously extended. Because the developing units with liquid developer40Y,40M,40C and40B have the same structure as to each other and are replaceable with each other, a desired color can be easily set for the single color mode by placing a developing unit with liquid developer of the desired color at the photoconductive element located at a foremost right end (at the side of a common image transfer area).

When the cleaning device79is positioned at a place shown inFIG. 2, i.e., at a tip end side of the pivot of the intermediate transfer belt100, the cleaning device509has to move along with the intermediate transfer belt100as indicated by a two-dotted and dashed line in FIG.2. Therefore, a load imposed on the belt position change mechanism110is increased and a distance the cleaning device509has to move is also increased, which may result in inconvenience of, for example, a leakage of developer etc. According to the embodiment of the present invention, because the cleaning device79is located at the base end side of the pivot of the intermediate transfer100, the increase of the load imposed on the belt position change mechanism110as well as the distance the cleaning device79moves are minimized, which may suppress inconvenience of the leakage of developer from the cleaning tank etc.

In the printer according to the embodiment of the present invention, either the black color mode (single color mode) or the multicolor mode is selectable. However in actuality, various modes with a combination of colors, such as 2 colors printing with black BK and cyan C colors, 3 colors printing with black BK, cyan C and magenta M colors and so forth, may be required. In order to cope with the requirement for various modes, a stepped belt position change mechanism112to change the position of spanning rollers75,76and80in steps as shown inFIG. 4may be employed to control a position of the intermediate transfer belt100. The stepped belt position change mechanism112functions to change the number of the photoconductive elements separating from the intermediate transfer belt100in steps and uplifts or lifts down spanning rollers75,76and80individually and independently. In the multicolor mode, for example, when a two colors mode with black color BK and cyan color C is set, the intermediate transfer belt100is brought into contact only with photoconductive elements10C and10B separating from photoconductive elements10Y and10M by uplifting the tension roller71and spanning rollers76and80while keeping the spanning roller75at a lifted down position as indicated by a chained line in FIG.4. Further, in the multicolor mode, for example, when three colors mode with black BK, cyan C and magenta M colors is set, the intermediate transfer belt100is brought into contact only with photoconductive elements10M,10C and10B separating from the photoconductive element10Y by uplifting the tension roller71and spanning roller80while keeping the spanning rollers75and76at the lifted down position as indicated by a two-dotted and dashed line in FIG.4. As a result, the positon of the intermediate transfer belt100can be controlled precisely so as not to contact photoconductive elements which are not involved in the image forming and printing operation which advantageously extends the life of photoconductive elements10Y,10M and10C.

Furthermore, the printer according to the embodiment of the present invention may be preferably configured such that mechanical devices (driving devices for the photoconductive elements and developing units) for the photoconductive elements which are separated from the intermediate transfer belt100(for example, photoconductive elements10Y,10M and10C in a case of the black color mode) are controlled to be stopped. By this control, the life of the photoconductive elements, developing units with liquid developer and its driving devices can be extended, and a consumption of electricity and a vibration can be reduced. Further, unnecessary consumption of developer through the unnecessary operation of the developing unit is avoided.

Further, in the printer according to the embodiment of the present invention, the intermediate transfer belt100is configured to partly pivot so as to separate from part of the photoconductive elements, however, it may be configured such that photoconductive elements are driven to uplift or lift down so as to separate from the intermediate transfer belt100. In this case, because the photoconductive elements, which are movable independently, change positions, the separation mechanism can be made simpler compared with the one in which the intermediate transfer belt100partly pivots by moving the above-described supporting rollers. Further, because the space for moving part of photoconductive elements is less than the one in which the intermediate transfer belt100partly pivots, it is also advantageous to reduce a size of the apparatus.

In the embodiment of the present invention, when a change in a tension of the intermediate transfer belt100occurs in the separation of the intermediate transfer belt100from part of the photoconductive elements, it is desirable to change a distance of at least one of the supporting rollers relative to the other supporting rollers. For example, the tension roller71may be configured to move toward the outside of the apparatus so as to suppress a change in the tension of the intermediate transfer belt100as explained in the following embodiment of the present invention. The intermediate transfer belt100can be driven accurately by the driving roller72by suppressing the change in the tension of the intermediate transfer belt100.

Now, an electrophotographic copying machine with liquid toner as an example of an image forming apparatus according to the another embodiment of the present invention is explained.

FIG. 5is a schematic drawing illustrating an internal construction of the copying machine. The copying machine has four sets of image forming sections1Y,1M,1C and1B, an intermediate transfer unit70which is detachable/attachable to a main body of the copying machine, a fixing device90, and an image reading unit (scanning unit), a sheet feeding unit and a controlling unit which are not shown.

The above four sets of image forming sections1Y,1M,1C and1B each includes photoconductive drums10Y,10M,10C and10B, developing units40Y,40M,40C and40B etc. Developing units40Y,40M,40C and40B use yellow toner, magenta toner, cyan toner and black toner respectively.

Eelectrostatic latent images of corresponding colors are formed on surfaces of corresponding photoconductive drums10Y,10M,10C and10B and are developed in respective developing units40Y,40M,40C and40B into toner images (visible images) with respective colors. The color toner images on the photoconductive drums are transferred to an intermediate transfer belt100being superimposed one after another, creating a multicolor toner image. Then, the multicolor toner image on the intermediate transfer belt100is transferred at one time to a transfer sheet200.

Because the four sets of image forming sections have the same construction, the image forming section1B will be described as an example of an image forming section.

The image forming section1B includes a photoconductive drum10B as an image bearing member, a charging device20B to uniformly charge a surface of the photoconductive drum10B, a laser writing unit30irradiating a laser beam light (LB), a liquid-type developing unit40B, a discharging device50B and a cleaning device60B having a cleaning blade. A visible image is formed on the photoconductive drum10B with the charging device20B, the laser writing unit30and the developing unit40B etc.

The liquid-type developing unit40B includes a developing roller41B as a developer carrier, a developer reservoir42B to store a developer, a developer scoop up roller43B provided so as to be immersed in liquid developer in the developer reservoir42B and a developer coating roller44B which laminates and coats the developer scooped up by the developer scoop up roller43B on the developing roller41B.

The liquid developer used in the liquid-type developing unit includes toner particles to make a latent image visible, which are dispersed at a high ratio in a carrier liquid and insulating material, having a viscosity as high as 100 to 10,000 mPa·s

The intermediate transfer unit70includes supporting rollers71,72,73,74,75,76,78and80, the intermediate transfer belt100(opposing member) which is spanned around those rollers, primary transfer bias rollers77B,77Y,77M and77C as primary transfer bias applying members and an intermediate transfer belt cleaning device79having a cleaning blade79a. The supporting roller72is connected to a driving means (not shown) and is configured to function as a drive roller also to rotatively drive the intermediate transfer belt100.

It is preferable that the intermediate transfer belt100is elastic at its surface contacting a transfer sheet without being elastic in a circumferential direction. Because the elastic surface of the intermediate transfer belt100is brought into intimate contact with the transfer sheet by adhering to a concave surface of the transfer sheet, a satisfactory transfer of the toner image onto the transfer sheet can be obtained.

As in the first embodiment the intermediate transfer belt100, may be configured to be a double layer construction, having a first layer including an elastic member formed on a surface side where a toner image formed and a second layer including a resin sheet is formed on a back side thereof, and having a volume resistivity of 107to 1012Ωcm may be used. For the first layer, a polyurethan rubber layer of 200 to 700 μm in thickness. And as for the resin sheet layer, a polyurethan resin sheet of 100 to 500 μm in thickness, which is not stretched in a circumferential direction, may be used. Further, the intermediate transfer belt100may include a combination of a first layer of rubber formed on the surface (e.g. a nitrile rubber, a urethan rubber, the Butyl-rubber and a natural rubber) and a second layer of a fiber buried rubber, or a combination of a first coated layer including a fluorine resin and a second layer of an elastic conductive element having the volume resistivity of 105to 109Ωcm. The elastic conductive element may include a polyurethan rubber with carbon dispersed.

When the intermediate transfer belt100is configured to have the thickness of 200 to 2000 μm, a volume resistivity of 105to 109Ωcm and a hardness of 15° to 80° in JIS A (Japanese Industrial Standards A), a specified effect will be obtained. The non-elastic core prevents the elastic conductive element from being stretched in the circumferential direction and it may include, for example, a nylon cord or a steel cord of 50 to 400 μm in diameter. The surface coated layer is provided to increase a transferability of a secondary transfer by improving a release of toner particles and to achieve a smoother separation of the transfer sheet200after the secondary transfer operation. The surface coated layer may include, for example, a layer including a fluorine resin coated in 5 to 60 μm thickness.

As for supporting rollers71-76and80, a grounded conductive roller (e.g. a metal roller) may be used. As for the primary transfer roller77and the secondary transfer roller81, a columned or cylindrical-shaped conductive roller (e.g. a metal roller or a metal pipe) having a conductive rubber layer (e.g. a hydrin rubber) on its surface may be used.

When the intermediate transfer belt100having a conductive layer on its underside is used, a floating state conductive roller (e.g. a metal roller) or a nonconductive roller is used for supporting rollers72-76and80other than the tension roller71and for the primary transfer roller77. The tension roller71is made of a conductive roller so that the conductive layer of the intermediate transfer belt100has a predetermined potential by a bias voltage applied to the tension roller71. When the transfer bias is applied to the secondary transfer roller81, a transfer electric field is formed by the potential difference between the conductive layer of the intermediate transfer belt100and the secondary transfer roller81.

A secondary transfer section to transfer a toner image formed on the intermediate transfer belt100to the transfer sheet200includes a secondary transfer roller81around which the intermediate transfer belt100windingly contacts and forms a secondary transfer nip therebetween and a secondary transfer power supply (not shown) as a transfer bias applying device, connected to the secondary transfer roller81.

The intermediate transfer belt100is windingly brought into contact with the photoconductive drums10B,10C,10M and10Y with specified contacting angles by the supporting rollers74,75,76,78and80(hereinafter referred to as spanning roller as necessary) which are located adjacent to respective photoconductive drums. The intermediate transfer belt100is spanned around a supporting roller71located at the left end inFIG. 5with the greatest contacting angle (hereinafter referred to as a tension roller as necessary) so as to maintain a specified belt tension. Further, the intermediate transfer belt100is rotatively driven in a counterclockwise direction indicated by an arrow by a supporting roller72(hereinafter referred to as a driving roller as necessary) opposite to a secondary transfer roller81located at the right end in FIG.5. The primary transfer bias roller77B is provided opposite to the photoconductive drum10B and the intermediate transfer belt100is interposed between the primary transfer roller77B and the photoconductive drum10B. The primary transfer roller77B also functions as an electrode applying a primary transfer bias while being applied with a specified primary transfer bias by a primary transfer power supply (not shown).

FIGS. 6 and 7illustrate locations of the intermediate transfer belt100in multicolor and black and white image forming processes respectively. In the multicolor image forming process shown inFIG. 6, the intermediate transfer belt100is supported by respective supporting rollers so as to contact the photoconductive drums10B,10Y,10M and10C with a specified contacting angle of θ.

In the black and white image forming process illustrated inFIG. 7, the intermediate transfer belt100moves to a position where the intermediate transfer belt100is separated from the photoconductive drums10Y,10M and10C while it remains in contact with only the photoconductive drum10B for black color, the drum closest to a secondary transfer area, located at the right end inFIG. 7. Aseparation device, for moving the intermediate transfer belt100to the separated position, pivotably moves a pivot subunit (not shown), to which shafts of the supporting rollers71,75,76and80and the primary transfer roller77Y,77M and77C are attached, about the spanning roller74located between the photoconductive drums10B and10C, by a pivot mechanism (not shown), in a clockwise direction as indicated by arrow A in FIG.7.

FIG. 8explains a pivot mechanism of the pivot subunit701which is part of the intermediate transfer unit70. The intermediate transfer unit70includes the pivotable pivot subunit701and a fixed subunit702. Spanning rollers75,76and80, and primary transfer rollers77Y,77M and77C are rotatably provided to a sideboard701aof the pivot subunit701. The primary transfer roller77B for black color, the driving roller72, the guide roller73and spanning rollers74and78are rotatably provided to a sideboard702aof the fixed subunit702. The pivot subunit701pivots about the shaft of the fixed spanning roller74. Above the spanning roller74, an oblong hole701bfor the pivot is provided on the sideboard701aso that a guide pin702bprovided to the fixed subunit702passes through the oblong hole701b. When the pivot subunit701pivots, the guide pin702bguides the pivoting of the pivot subunit701.

FIG. 9illustrates a driving section of the pivot mechanism to pivot the pivot subunit701. The driving section includes a timing belt706in an endless form spanned around pulleys704and705. A shaft704aof the pulley704is rotatably supported by a main body of the apparatus. The pulley705is connected to a rotation shaft of a motor707that is supported by the main body of the apparatus. The motor707can reverse the direction of rotation and is controlled by an engine control CPU (central processing unit) described later. A fixing member703is provided at a spanned portion of the timing belt706between pulleys704and705so as to sandwich support the timing belt706. The fixing member703is fixed to the sideboard701aof the pivot subunit701.

In the above-described driving section, when the motor701rotates in a normal or reverse direction, the fixing member703moves in a vertical direction (in a direction indicated by a double-headedd arrow H inFIG. 9) along with the movement of the timing belt706. By the movement of the fixing member703, the pivot subunit701, to which the fixing member703is fixed, pivots as indicated by an arrow I in FIG.9.

When the intermediate transfer belt100is moved to the separated position, the intermediate transfer belt100is slackened and a tension of the intermediate transfer belt100tends to be reduced. Therefore, a relative distance change device is provided to move the tension roller71in a direction (the direction indicated by an arrow B inFIG. 7) that increases a relative distance of the tension roller71and the other supporting rollers when the above mentioned supporting rollers etc. are rotatively moved. The movement of the tension roller71prevents the tension of the intermediate transfer belt100from lowering. Positions of parts designated with a dash (′) inFIG. 7(and inFIG. 10) show virtual intermediate positions of the corresponding parts when they are moved.

FIGS. 10 and 11are expanded sectional and perspective views respectively illustrating an example of a tension roller driving mechanism as the relative distance changing device according an embodiment of the present invention. The tension roller driving mechanism includes a biasing member that moves together with the tension roller71and applies a resilient bias to a bearing71afor the tension roller71so that the tension roller71press-contacts the intermediate transfer belt100. The tension roller driving mechanism also includes a fixed guide member103which thrusts an other end of a junction member102to move the biasing member toward the tension roller71. The biasing member includes a spring101, an end of which touches the bearing71aof the tension roller71and the junction member102performs a reciprocating motion being thrusted by an other end of the spring101. The junction member102includes two oblong holes12aand pins104attached to the side of the pivot unit through the oblong holes12a. The junction member102performs reciprocating motion while being supported by the pins104and pivots together with the tension roller71.

The fixed guide member103is fixed to a body of the image forming apparatus and includes recesses103aand103bwhere an end of the junction member102is engagedly held temporarily in the multicolor and the black and white image forming processes respectively as illustrated in FIG.12. Because the end of the junction member102is engagedly held with the recesses103aor103bof the fixed guide member103, the end of the junction member can be held firmly in respective positions that stabilizes the tension of the intermediate transfer belt100maintained by the junction member102via the spring101.

For the fixed guide member103, a resin that possesses a low coefficient of friction such as polyacetal, polycarbonate and polyamide is preferable. Because a friction produced when the end of the junction member102moves in contact with a surface of the fixed guide member103is lowered, a load imposed on the pivot of the pivot subunit701, which includes part of the above mentioned supporting rollers, is decreased.

For the biasing member to apply a resilient bias to the bearing71aof the tension roller71, a set of cylindroid members105and106with different diameters, which are configured such that one cylindroid member moves back and forth through the other cylindroid member having a spring107in it as illustrated in FIG.13. An end of the cylindroid member105is attached to the bearing71aof the tension roller71. The other cylindroid member106is fixed to the pivot subunit701so as to perform a reciprocating movement and to contact the fixed guide member103at its end.

As illustrated inFIG. 14, the cleaning unit79including a cleaning blade79aand a cleaning roller79bis configured to move integrally with a bearing71aof the tension roller71. Accordingly, even when the tension roller71is moved in a direction indicated by an arrow B inFIG. 14, the cleaning blade79aand the cleaning roller79bof the cleaning device79securely contact the intermediate transfer belt100, and thereby a satisfactory cleaning performance for the intermediate transfer belt100is maintained.

FIG. 15is a block diagram explaining a data process control system of the copying machine according to embodiment of the present invention. Image data transmitted from a scanning device is converted to image data corresponding to respective colors at an image data processing section124and is stored in page memories121Y,121M,121C and121B corresponding to respective colors of yellow, magenta, cyan and black. The mode determination circuit122determines a single color mode (black color mode) or a multicolor mode based on the image data output from each page memory. The engine control CPU123controls a driving device113for the pivot subunit701etc. according to a result of an image forming mode discrimination at the mode discrimination circuit122. By this control, unnecessary contact of the intermediate transfer belt100with the photoconductive elements10Y,10M and10C which are not used and the change in the tension of the intermediate transfer belt100can be avoided according to the determined image forming mode. Especially, when the image forming operation is switched from the black color mode to the multicolor mode, it is preferable that the apparatus is controlled such that the pivot subunit701pivots and rotatively drives the intermediate transfer belt100and cleans the intermediate transfer belt100two or more times by the cleaning device79utilizing a time when image data for the multicolor image forming is processed. By this control, a time for the copying machine to start the image forming operation after a copy start button is pressed is shortened and the cleaning performance for the intermediate transfer belt100is enhanced without taking an additional time for the cleaning.

Next, an image forming operation of the copying machine will be described. As illustrated inFIG. 5, a surface of the photoconductive drum10B is uniformly charged with a charging device20B while the photoconductive drum10B is rotating in a direction indicated by an arrow. Then, an electrostatic latent image is formed on the surface of the photoconductive drum10B being exposed to a laser light beam LB irradiated from the laser writing unit30. The developing roller41B is uniformly coated, for example, in the thickness of about 0.5 to 20 μm, via the developer applying roller44B with liquid developer adhered to the developer scoop up roller43B which is immersed in high-viscosity liquid developer in the developer reservoir42B. The developing roller41B is brought into contact with the photoconductive drum10B so that toner in liquid developer is applied to the latent image formed on the surface of the photoconductive drum10B by virtue of an electric field, and thereby a visible toner image is formed.

The toner image formed on the photoconductive drum10B is moved to a primary transfer area along with the rotation of the photoconductive drum10B where the photoconductive drum10B abuts against the intermediate transfer belt100. In the primary transfer area, a back of the intermediate transfer belt100is applied with a negative bias voltage of, for example, −300 to −500, through the primary transfer bias roller77B. Then the toner of the toner image formed on the photoconductive drum10B is attracted to the intermediate transfer belt100by a force of an electric field generated by the applied voltage to transfer the toner image to the intermediate transfer belt100(a primary transfer). The toner image is formed on the intermediate transfer belt100in order of yellow, magenta, cyan and black, and the toner images of respective colors are transferred to the intermediate transfer belt100superimposed one after another to form a full color image (visible image).

The intermediate transfer belt100having the multicolor toner image travels to a secondary transfer area where the intermediate transfer belt100abuts against a transfer sheet200conveyed from a sheet feeding unit (not shown) in a direction indicated by an arrow in FIG.5. In the secondary transfer area, a back of the transfer sheet200is applied with a negative bias voltage of, e.g., −800 to −2000 through the secondary transfer roller81, which presses the transfer sheet200with a force of about 50 N/cm2. The toner on the intermediate transfer belt10is attracted and transferred onto the transfer sheet200at one time by virtue of an electric field generated by the application of the voltage and the pressure exerted to the transfer sheet200(a secondary transfer).

The transfer sheet200carrying the transferred toner image is separated from the intermediate transfer belt100by a transfer sheet separation member91and is discharged to an exit tray after the toner imager is fixed onto the transfer sheet200by a toner image fixing device90. After the secondary transfer operation, the surface of the photoconductive drum10B is uniformly discharged by a discharging device50B and is cleaned by a cleaning device60B and remaining residual toner is removed to be ready for a next image forming operation.

When a black and white image is formed in the above configured copying machine, as illustrated inFIG. 7, the pivot subunit (not shown) disposed at the side of a color image forming section pivots while an image forming operation is not performed such that the intermediate transfer belt100moves to the separated position where the intermediate transfer belt100remains in contact only with the photoconductive drum10B for black color which is the closest drum to the secondary transfer area, (disposed at the right side end inFIG. 7) while being separated from the other photoconductive drums10Y,10M and10C. A toner image is formed only on the surface of the photoconductive drum10B and is then transferred to the intermediate transfer belt100. The toner image on the intermediate transfer belt100is then transferred onto the transfer sheet200at the secondary transfer area to form a black and white image on the transfer sheet200.

According to the embodiment of the present invention, even when the intermediate transfer belt100is tentatively separated from the three photoconductive drums10Y,10M and10C for the multicolor image forming process in a black and white image forming operation, a change in the intermediate transfer belt100is suppressed and thereby the intermediate transfer belt100is frictionally driven accurately. Thus a quality degradation of a produced image caused by a deviation of the image position or image size etc. is suppressed.

According to the embodiment of the present invention, the tension roller71, with which the intermediate transfer belt100is in contact with the largest contacting angle among the supporting rollers, moves when the intermediate transfer belt100moves to the separated position.

Generally, the larger the contacting angle of the intermediate transfer belt100with a supporting roller is, the larger the amount of a change in a circumferential length of the intermediate transfer belt100relative to a unit of travel of the supporting roller is. For example, when a contacting angle (θ) of the intermediate transfer belt100with a supporting roller700is 180°, the amount of a change (Δl) in the circumferential length of the intermediate transfer belt100is 2D when the supporting roller700is moved by a distance of D toward the outside of the apparatus as indicated by an arrow B in FIG.16A. Contrarily, as shown inFIG. 16B, when the contacting angle (θ) of the intermediate transfer belt100with the supporting roller70is less than 180°, the amount of a change (Δl in a circumferential length of the intermediate transfer belt100is less than 2D even when the supporting roller700is moved toward the outside of the apparatus by the same distance of D described in FIG.16A.

In this embodiment, because the tension roller71, with which the intermediate transfer belt100is in contact and which has the largest contacting angle among the supporting rollers, is moved, the amount of movement of the tension roller71to prevent the tension of the intermediate transfer belt100from being decreased is minimized.

Further, the amount of a movement of the tension roller71is set such that the intermediate transfer belt100is spanned around a plurality of supporting rollers while being tensioned when the intermediate transfer belt100is pivoted such that, referring toFIG. 17, a sum of (1) a length of of the intermediate transfer belt100windingly in contact with a plurality of contacting members such as the supporting rollers etc. and (2) a non-contacting length of the intermediate transfer belt between contacting members where the intermediate transfer belt100is not in contact with any contacting member, does not change. As illustrated inFIG. 17, the contacting length is the length of the intermediate transfer belt100windingly in contact with contacting members602and603, which is indicated by L1, and the non-contacting length is the length of the intermediate transfer belt100spanned straightly between contacting members602and603where the intermediate transfer belt100does not contact any contacting member, which is indicated by L2. In this embodiment, contacting members602and603correspond to supporting rollers and photoconductive elements.

The change in the tension of the intermediate transfer belt100is securely suppressed by setting the amount of the movement of the tension roller71as described above.

In the above-described embodiment of the present invention, the intermediate transfer belt100is configured to partly pivot so as to separate from part of photoconductive elements10Y,10M,10C and10B, however, as illustrated inFIG. 18, part of photoconductive elements10Y,10M and10C may be configured to be brought down so as to be separated from the intermediate transfer belt100. The change in the tension of the intermediate transfer belt100can be suppressed by moving the tension roller71, along with the separating movement, by a specified distance D in a direction of a tension applied to the intermediate transfer belt100.

A mechanism to move the photoconductive elements can be simpler compared with the one that partly pivots the intermediate transfer belt100as described above. It is also advantageous in reducing the size of the apparatus because the mechanism to move the photoconductive elements requires less space than the one to move the intermediate transfer belt100.

An eccentric cam109may be employed in a mechanism to move the tension roller71as illustrated inFIGS. 19A and 19B. The eccentric cam109is rotated about 90° i.e., from a state illustrated inFIG. 19Ato a state inFIG. 19Bso as to move the tension roller71by thrusting the bearing71athrough a spring101. Especially, when the eccentric cam109is employed, because the tension roller71can be moved in multiple steps by adjusting the angle of the rotation of the eccentric cam109, an adjustment of the tension of the intermediate transfer belt100can be easily made.

FIG. 20is a block diagram explaining a data process control system of the image forming apparatus (a printer) configured to move the tension roller71by the eccentric cam109. In the image forming apparatus, the driving device114for the eccentric cam109and the driving device113for the pivot subunit701are controlled according to a result of an image forming mode discrimination. By this control, unnecessary contact of the intermediate transfer belt100with photoconductive elements and a change in the tension of the intermediate transfer belt100are securely avoided in response to the determination of the image forming mode.

As illustrated inFIGS. 21 and 22, the photoconductive element10B for black color may be located in a different level in a dirrection orthogonal to the axes of photoconductive elements10Y,10M and10C. To be specific, as illustrated inFIG. 21, photoconductive elements10Y,10M and10C are disposed such that a center line of photoconductive elements10Y,10M and10C (indicated by a chained line C1) is located further from the intermediate transfer belt100than a center line of the photoconductive element10B (indicated by a chained line C2), which is in parallel with C1, by a level difference of E. As illustrated inFIG. 23, which is a view from a direction indicated by an arrow F inFIG. 21, in this configuration the tension roller71acts to correct shifting of the intermediate transfer belt10to one side. One end71bof a shaft of the tension roller71is fixed to a housing70aof the intermediate transfer unit70and the eccentric cam710abuts against the other end71cof the shaft via a bearing. The end71cof the shaft moves in a direction (vertical direction indicated by a double-headed arrow G) orthogonal to a direction to which a tension is applied to the intermediate transfer belt100so as to correct the shifting of the intermediate transfer belt100to a width direction.

A chained line and a two-dotted and dashed line in the proximity of the intermediate transfer belt100(a solid line) inFIGS. 21 and 22illustrates edges of the intermediate transfer belt100when the intermediate transfer belt100is moved by the tension roller71to correct a shifting of the intermediate transfer belt100in the width direction.

The cleaning device79to clean a surface of the intermediate transfer belt100is configured to move integrally with the tension roller71(see FIG.14). Therefore, even when the tension roller71changes its position to correct a shifting of balance of the intermediate transfer belt100, the cleaning blade79aand the cleaning roller79bsecurely contact the intermediate transfer belt100, and thereby the intermediate transfer belt100is kept well-cleaned.

In this configuration, when the intermediate transfer belt100is separated from the photoconductive elements10Y,10M and10C in the black color mode, positions of the spanning rollers78and78′ and the primary transfer roller77B relating to the photoconductive element10B remain unchanged as illustrated in FIG.22. Alternatively, spanning rollers74,75,76and80, and primary transfer rollers77Y,77M and77C relating to photoconductive elements10Y,10M and10C are moved in an upward direction, separating from these photoconductive elements, by a driving mechanism (not shown). Thus, the intermediate transfer belt100can be separated from photoconductive elements10Y,10M and10C by moving only part of the spanning rollers and primary transfer rollers.

In the above-described separation of the intermediate transfer belt from the photoconductive elements, supporting rollers82and83for applying a supplementary pressure to the intermediate transfer belt100(hereinafter referred to as supplementary roller) are moved in an upward direction to press an underside of the portion of the intermediate transfer belt100spanned between the driving roller72and the tension roller71so as to prevent the tension of the intermediate transfer belt100from changing (a decrease in the tension). Further, in this configuration, the tension roller71is not required to be moved greatly in order to suppress the change in the tension of the intermediate transfer belt100caused by the above-described separation of the intermediate transfer belt100from photoconductive elements. Therefore, the conditions of the tension of the intermediate transfer belt100given by the tension roller71, and the function of the tension roller71to correct a shifting of the intermediate transfer belt100are hardly influenced by the separation of the intermediate transfer belt100from photoconductive elements, thus making it possible to maintain the quality of images.

As illustrated inFIG. 21, in the multicolor mode where the intermediate transfer belt100contacts photoconductive elements10Y,10M and10C, supplementary rollers82and83are located so as to securely separate from the underside of the intermediate transfer belt100even when maximum shifting correction is made to the intermediate transfer belt by the tension roller71. Consequently, in the multicolor mode, the function of the tension roller71to correct a shifting of the intermediate transfer belt100may not be affected by a contact of supplementary rollers82and83with the intermediate transfer belt100.

In the above described embodiment of the present invention, a belt-formed member and an opposing member which contacts the belt-formed member are described as the intermediate transfer belt100and the photoconductive drums respectively. However, the present invention can also be applied when the belt-formed member is a photoconductive belt300and a plurality of opposing members, contacting the photoconductive belt300, are developer rollers41B,41Y,41M and41C, as illustrated in FIG.24.

In the image forming apparatus illustrated inFIG. 24, charging devices305B,305Y,305M and305C are disposed to oppose supporting rollers304B,304Y,304M and304C at an upstream side of respective developing rollers in the moving direction of the photoconductive belt300. Opposing rollers307B,307Y,307M and307C are provided at positions opposed to developing rollers41B,41Y,41M and41C respectively while the photoconductive belt300is interposed between the opposing rollers and the developing rollers. The photoconductive belt300is uniformly charged by the charging devices305B,305Y,305M and305C and is exposed to laser beam lights corresponding to colors of an original image from a laser writing unit and then electrostatic latent images corresponding to respective colors are formed on the photoconductive belt300. When a black and white image is formed in the image forming apparatus, supporting rollers301,304Y,304M and304C and opposing rollers307Y,307M,307C as well as the photoconductive belt300are pivoted about the supporting rollers304B located between developing rollers41B and41C in a direction indicated by an arrow A in FIG.24. Then, the photoconductive belt300is separated from developing rollers41Y,41M and41C. During the pivotal movement, the supporting roller301, which also works as a tension roller, moves toward the outside of the apparatus as indicated by an arrow B inFIG. 24so as to prevent a tension of the photoconductive belt300from decreasing, thus enabling the photoconductive belt300to be driven accurately even in the black and white image forming operation.

Especially, in the configuration illustrated inFIG. 24, the photoconductive belt300and the belt-formed member may be disposed contacting or in the vicinity of developing rollers41B,41Y,41M and41C as a plurality of opposing members (developer bearing member). The arrangement of the photoconductive belt300and developing rollers41B,41Y,41M and41C can be determined according to a development system such as contacting and non-contacting development systems. The present invention can be applied to both developing systems.

Further, as illustrated inFIG. 25, the present invention can also be applied to an image forming apparatus configured such that a belt-formed member is a transfer sheet conveying belt400to convey a transfer sheet200to a transfer area while a plurality of opposing members opposed to the transfer sheet conveying belt400are photoconductive drums10B,10Y,10M and10C of respective colors. In the image forming apparatus illustrated inFIG. 25, the transfer sheet conveying belt400is supported by a plurality of supporting rollers401,402,403and404and charging devices405B,405Y,405M and405C are arranged opposing to respective photoconductive drums10B,10Y,10M and10C while interposing the transfer sheet conveying belt400between the charging devices and the photoconductive drums. Supporting rollers401and403serve as a belt driving roller and a tension roller respectively.

When a black and white image is formed in the image forming apparatus, the supporting roller (the tension roller)403as well as charging devices405Y,405M and405C are pivoted about the supporting roller404located between photoconductive drums10B and10C in a direction indicated by an arrow A in FIG.25. Thereby the transfer sheet conveying belt400is separated from the photoconductive drums10Y,10M and10C. In the pivotal movement, the supporting roller403, which also functions as a tension roller, is moved toward the outside of the apparatus as indicated by an arrow B to prevent a tension of the transfer sheet conveying belt400from decreasing, thus enabling the transfer sheet conveying belt400to be frictionally driven accurately even in the black and white image forming operation.

The present invention may be also applied to an image forming apparatus configured such that a tension of a belt-formed member is increased when the belt-formed member separates from some of the opposing members as illustrated in FIG.26. The image forming apparatus shown inFIG. 26is configured in a manner similar to the apparatus illustratedFIG. 4, however, a pivot of a pivot subunit including part of supporting rollers71,75,76and80is positioned differently. In the image forming apparatus shown inFIG. 26, a pivot601is positioned such that a tension of the intermediate transfer belt100is increased in the above described pivotal movement.

When a black and white image is formed in the image forming apparatus, part of supporting rollers77Y,77M and77C are pivoted about the pivot601in a direction indicated by an arrow A in FIG.26. Thereby, the intermediate transfer belt10is separated from the photoconductive drums10Y,10M and10C. During the pivotal movement, the supporting roller71, which also functions as a tension roller, moves toward the inside of the apparatus as indicated by an arrow B inFIG. 26to prevent the tension of the intermediate transfer belt100from being increased which consequently suppresses a driving load from increasing and enables the intermediate transfer belt100to be frictionally driven accurately even in the black and white image forming operation.

The positions of the supporting rollers designated with a dash (') inFIG. 26indicate virtual intermediate positions of corresponding rollers when they are moved.

In the above described embodiments of the present invention, the description has been made for the image forming apparatus using high viscosity liquid developer, however, the present invention can also be applied to image forming apparatuses using dry developer or liquid developer other than the high viscosity developer.

Further, in the above-described embodiments of the present invention, a belt-formed member such as an intermediate transfer belt is described in an endless form, however, the present invention may be applied to belts other than such an endless belt and produces the same effect. For example, it can be applied to a configuration in which a belt supplied from a supplying roller is driven so as to be wound up by a winding roller. In this configuration, for example, the belt is supported by a plurality of supporting rollers with a constant tension such that a portion of the belt spanned around the reel roller and the supplying roller opposes a plurality of opposing members. A route that the belt is spanned is changed so as to separate from part of the opposing members when necessary. In the separating operation, relative distances between the supporting rollers are adjusted so as to suppress the change in the tension of the belt.

Moreover, in the above-described embodiments, the description has been made with respect to image forming apparatuses, however the present invention can be applied to a belt device including a belt-formed member supported by a plurality of supporting rollers and a plurality of opposing members which are located opposite to the belt-formed member and side by side in a line, contacting the belt-formed member or in the vicinity of the belt-formed member. According to the present invention, unnecessary contact of the opposing members with the belt-formed member is suppressed and thereby decrease of the life of the opposing member is avoided.