Transfer belt unit and image forming apparatus using the same

A transfer belt unit and image forming apparatus that achieves the reduction in size and cost of a transfer belt unit and can prevent fluctuation of the cleaning performance. The unit has a transfer belt, a plurality of rollers around which the transfer belt is laid with certain tension, wherein at least one of the rollers is a tension roller for tensioning to the transfer belt; frames supporting the plurality of rollers; tensioning mechanisms which are disposed on the side faces of the frames to bias the tension roller in a belt tensioning direction; a cleaning mechanism in contact with the transfer belt at a position opposing the tension roller; rotation preventing members which prevent the rotational torque form being transmitted to the cleaning mechanism; and position restricting members which restrict the relative position between the cleaning mechanism and the tension roller.

BACKGROUND OF THE INVENTION

The present invention relates to a tandem-type image forming apparatus in which image forming stations for respective colors are arranged along a transfer belt, each image forming station being composed of an image carrier, a charging means and a developing means which are arranged around the image carrier. The image forming apparatus forms a multi-color image by passing the transfer belt through every station.

Tandem-type image forming apparatuses as described above are categorized into two types as:

(1) an apparatus employing a paper delivery method which comprises a plurality of image forming stations arranged in an array, in which a receiving medium is electrostatically attracted to a delivery belt and is fed to be brought in contact with the respective stations in order and electrostatic transferring force is applied between each station and the recording medium, thereby superposing toner images of plural colors while directly transferring the toner images to the receiving medium; and

(2) an apparatus employing an intermediate transfer method which comprises a plurality of image forming stations arranged in an array, in which an intermediate transfer belt made of a dielectric substance is fed to be brought in contact with the respective stations and electrostatic transferring force is applied between each station and the intermediate transfer belt so as to transfer primarily toner images of the respective stations one by one to superpose the toner images on the intermediate transfer belt and the superposed toner images are transferred secondarily from the intermediate transfer belt to a recording medium at once.

In the aforementioned paper delivery method, it is required to provide a means (roller or brush) for attracting the receiving medium to the delivery belt and high voltage power supply. In the intermediate transfer method, however, such a means and high voltage power supply are not required. Further, in the paper delivery method, it is required to strictly control the transfer bias to be applied to respective image transferred portions according to the size, the thickness, and the kind of the receiving medium. In the intermediate transfer method, the primary transfer of toner images is conducted to the intermediate transfer belt of which resistance, thickness, and surface roughness are constant regardless of the aforementioned factors of the receiving medium. The control of the transfer condition including the transfer voltage or transfer current and contact pressure must be conducted only for the secondary transfer of the toner images to the receiving medium. Therefore, the intermediate transfer method has a lot of advantages.

It is required to provide a cleaning means for removing toner remaining on the surface of the transfer belt after the transfer. Normally, a back-up roller is disposed on the back of the transfer belt to oppose a cleaning blade as the cleaning means. If using a belt tension roller as the back-up roller, the reduction in size and cost of the apparatus can be achieved. However, since the position of the belt tension roller is not constant because of allowable error in length of the transfer belt and allowable variance in tension, the contact position of the cleaning blade is varied and the cleaning performance is therefore not stable.

The first object of the present invention is therefore to provide a transfer belt unit and an image forming apparatus using the same, capable of resolving the aforementioned conventional problems, which can achieve the reduction in size and cost and can prevent the cleaning performance from being varied.

In the both cases of the intermediate transfer method and the paper delivery method, there is another problem that the accuracy of color registration may be deteriorated due to meandering of the intermediate transfer belt or the paper delivery belt (hereinafter, both referred to as “transfer belt”). The meandering of the transfer belt results from difference in peripheral length between both sides of the transfer belt or variation in material of belt which may be generated in the manufacturing process, or parallelism between a driving roller and a driven roller for the transfer belt.

Though various devices for controlling the meandering of the belt have been proposed, any of them is complex in structure.

The second object of the present invention is therefore to provide a transfer belt unit and an image forming apparatus using the same, capable of resolving the aforementioned conventional problems, which can prevent the meandering of a transfer belt with a simple structure.

SUMMARY OF THE INVENTION

For achieving the aforementioned first object, a transfer belt unit of the present invention is characterized by comprising: a transfer belt, a plurality of rollers around which the transfer belt is laid with certain tension, wherein at least one of the rollers is a tension roller for tensioning to the transfer belt; frames supporting the plurality of rollers; tensioning mechanisms which are disposed on side faces of said frames to bias said tension roller in a belt tensioning direction; a cleaning means being in contact with the transfer belt at a position opposing said tension roller; rotation preventing members which prevents the rotational torque from being transmitted to said cleaning means; and position restricting members which restrict the relative position between said cleaning means and said tension roller.

The transfer belt unit is further characterized in that each of said tensioning mechanisms comprises an elongate hole which is formed in each frame and through which a stationary shaft of the tension roller is inserted, a turning lever which is rotatably disposed on the frame and has one end being in contact with said stationary shaft, and a spring which is disposed between the other end of the turning lever and the frame.

The transfer belt unit is further characterized in that each of said rotation preventing member comprises a fitting flange which is formed in said frame to extend in the belt tensioning direction, and a receiving groove which is formed in the cleaning means and is fitted with said fitting flange.

The transfer belt unit is still characterized in that each of said position restricting members is a plate connecting a cleaner casing of said cleaning means and the stationary shaft of the tension roller.

An image forming apparatus of the present invention is characterized by comprising image forming stations for respective colors arranged along a transfer belt unit as mentioned above, each image forming station including an image carrier, a charging means and a developing means disposed around said image carrier, wherein the transfer belt is passed through the respective image forming stations, thereby forming a multi-color image.

The image forming apparatus is further characterized in that said transfer belt is an intermediate transfer belt.

The image forming apparatus is further characterized in that said transfer belt is laid around a driving roller and the tension roller with certain tension, said tension roller is positioned obliquely above the driving roller and is disposed such that the belt tension side at the time of driving the transfer belt is on the lower side, and the image carriers of the respective image forming stations are in contact with the transfer belt on the belt tension side.

The image forming apparatus is furthermore characterized in that said cleaning means is positioned on said belt tension side.

The image forming apparatus is still further characterized in that a toner collecting container is arranged along the side surface of one of said frames.

For achieving the aforementioned first object, a transfer belt unit of the present invention is characterized by including a meandering prevention means which comprises: a roller body around which a transfer belt is wrapped; a pair of retaining flanges disposed at both sides of said roller body; a flange portion formed on one of said retaining flanges to project from the outer periphery of said roller body; a slidable flange slidably arranged in the other retaining flange; a spring which biases said slidable flange toward said one retaining flange; and a flange portion formed on said slidable flange to project from the outer periphery of said roller body, wherein both side edges of a transfer belt are restrained between the flange portion of said one retaining flange and the flange portion of said slidable flange.

The transfer belt unit is further characterized in that said other retaining flange has a concave portion formed therein and said slidable flange has a convex portion, and wherein said convex portion is fitted in said concave portion and said spring is partially disposed in said other retaining flange.

The transfer belt unit is further characterized in that said one retaining flange is made of a non-conductive and low sliding resistance resin, the other retaining flange is made of conductive and low sliding resistance resin, and said other retaining flange is grounded.

The transfer belt unit is further characterized in that said slidable flange and said spring are arranged in at least one of a driving roller and a driven roller.

The transfer belt unit is still characterized in that said slidable flange and said spring are arranged in the driven roller and the diameter of the driven roller is set to be larger than that of the driving roller.

The transfer belt unit is further characterized in that said transfer belt is a belt which is made by an extrusion moulding method and has extrusion traces formed in the inner surface of the belt along the width direction of the same.

The transfer belt unit is furthermore characterized in that said transfer belt is a belt which is made by a centrifugal moulding method and has a layer of a low frictional resistance material which is formed on the back of the belt.

The transfer belt unit is still further characterized in that the belt is formed to have side edges thicker than the middle portion thereof.

An image forming apparatus of the present invention is an image forming apparatus of a tandem type comprising image forming stations for respective colors arranged along a transfer belt unit, each image forming station including an image carrier, a charging means and a developing means disposed around said image carrier, wherein the transfer belt is passed through the respective image forming stations, thereby forming a multi-color image, and is characterized in that said transfer belt is laid around a driving roller and a driven roller and at least one of said driving roller and a driven roller employs a meandering prevention means as mentioned above.

The image forming apparatus is further characterized in that said transfer belt is a paper delivery belt or an intermediate transfer belt.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG.1andFIG. 2show an embodiment of the image forming apparatus to which the present invention is adopted, whereinFIG. 1is a schematic sectional view showing the entire structure thereof andFIG. 2is an enlarged view of main parts in FIG.1. This embodiment is of a type employing the intermediate transfer method as described above.

InFIG. 1, the image forming apparatus1of this embodiment comprises a housing2, an outfeed tray2aformed in the top of the housing2, and a door body3which is attached to the front of the housing2in such a manner that the door body3is able to open or close freely. Arranged within the housing2are a control unit4, a power source unit5, an exposure unit (exposure means)6, an image forming unit7, a cooling means8composed of an air fan, a transfer belt unit9, and a paper feeding unit10. Arranged within the door body3is a paper handling unit11. The respective units are designed to be detachable relative to the housing2, whereby these units can be temporally detached for the purpose of repair or replacement for the time of maintenance.

The transfer belt unit9comprises a driving roller12which is disposed in a lower portion of the housing2and is driven by a driving means (not shown) to rotate, a driven roller13which is disposed diagonally above the driving roller12, an intermediate transfer belt14which is laid around the two rollers with certain tension and is driven to circulate in a direction indicated by an arrow (the counter-clockwise direction), and a cleaning means15which abuts on the surface of the intermediate transfer belt14. The driven roller13and the intermediate transfer belt14are arranged obliquely to the upper left of the driving roller12in FIG.1. Accordingly, during the operation of the intermediate transfer belt14, a belt face14aof which traveling direction Y is downward takes a lower side and a belt face14bof which traveling direction is upward takes an upper side. In this embodiment, the belt face14ais a tension side (a side tensioned by the driving roller12) at the time of driving the intermediate transfer belt14and the belt face14bis a slack side at the time of driving the intermediate transfer belt14.

The driving roller12also functions as a back-up roller for a secondary transfer roller39described later. As shown inFIG. 2, formed on the peripheral surface of the driving roller12is a rubber layer12awhich is 3 mm in thickness and 105Ω·cm or less in volume resistivity. The driving roller12has a metallic shaft which is grounded so as to function as a conductive path for secondary transfer bias supplied through the secondary transfer roller39. Since the driving roller12is provided with the rubber layer12ahaving high friction and shock absorption, impact generated when a receiving medium is fed into a secondary transfer section is hardly transmitted to the intermediate transfer belt14, thereby preventing the deterioration of image quality.

In this embodiment, the diameter of the driving roller12is set to be smaller than the diameter of the driven roller13. This facilitates the separation of a receiving medium after secondary transfer because of the elastic force of the receiving medium itself. The driven roller13also functions as a back-up roller for the cleaning means15described later.

The cleaning means15is located at the belt face14aside, of which traveling direction is downward. As shown inFIG. 2, the cleaning means15comprises a cleaning blade15afor removing toner remaining on the surface of the intermediate transfer belt14after the secondary transfer, and a toner carrying member15bfor carrying collected toner. The cleaning blade15ais in contact with the intermediate transfer belt14at a position where the intermediate transfer belt14is wrapped around the driven roller13.

On the back of the intermediate transfer belt14, primary transfer members16composed of leaf spring electrodes are disposed. The primary transfer members16are pressed into contact with the back of the intermediate transfer belt14by their elastic force at locations corresponding to image carriers17of respective image forming stations Y, M, C, and K, described later. A transfer bias is applied to each primary transfer member16.

The image forming unit7comprises the image forming stations Y (for yellow), M (for magenta), C (for cyan), and K (for black) for forming multi-color images (in this embodiment, four-color images). As clearly shown inFIG. 2, each image forming station Y, M, C, K has an image carrier17composed of a photosensitive drum, a charging means19composed of a corona charging means, and developing means20which are arranged around the image carrier17. It should be understood that the image forming stations Y, M, C, K may be arranged in any order.

The image forming stations Y, M, C, K are disposed such that the respective image carriers17are in contact with the belt face14a, of which traveling direction is downward, of the intermediate transfer belt14. As a result of this, the image forming stations Y, M, C, K are arranged in an obliquely leftward direction relative to the driving roller12in FIG.2. Each image carrier17is driven to rotate in the traveling direction of the intermediate transfer belt14as indicated by arrows. It should be noted that the intermediate transfer belt14may be arranged in an obliquely rightward direction relative to the driving roller12. In this case, the belt traveling direction Y should be the counter direction and the belt face of which traveling direction is downward should be the surface14b.

The exposure means6is disposed in a space formed obliquely below the image forming unit7which is arranged obliquely. The control unit4and the power source unit5are disposed in a space above the exposure means6. The paper feeding unit10is disposed below the exposure means6and at the bottom of the housing2. Since the control unit4and the power source unit5are arranged adjacent to the exposure means6, this arrangement can reduce the area for placing as compared to a case in which these are arranged in parallel to the frame supporting the components of the apparatus.

As shown inFIG. 1, the exposure means6comprises a polygon mirror motor21, a polygon mirror22, a single f-θ lens23, and a reflection mirror24. In addition, four reflective mirrors25are disposed above the reflection mirror24to make scanning lines for the respective colors parallel to the belt face14aand three reflective mirrors25are further disposed to aim the scanning lines, reflected by the reflective mirrors25, to the image carriers17.

By providing the reflective mirrors25, the scanning lines are bent, thereby shortening the height of the exposure means6and thus making the apparatus compact. The reflective mirrors25are arranged in such a manner as to make the respective lengths of the scanning lines to the image carriers17of the image forming stations Y, M, C, K equal to each other.

In the exposure means6having the aforementioned structure, image signals corresponding to the respective colors are formed and modulated according to the common data clock frequency and are then radiated from the polygon mirror22. The radiated image signals are aimed to the image carriers17of the image forming stations Y, M, C, K via the f-θ lens23, the reflection mirror24, and the reflective mirrors25, thereby forming latent images.

In this embodiment, the scanning optical system is arranged at a lower side of the apparatus, thereby minimizing the vibration of the scanning optical system due to vibration of the driving system of the image forming means which affects the frame supporting the apparatus and thus preventing the deterioration of image quality.

The cooling means8composed of an air fan is provided on one side of the housing2to introduce atmosphere in a direction of arrows in order to cool the exposure means6, the control unit4, and the power source unit5. Atmosphere drawn inside the apparatus from the rear side in the feeding direction of receiving media P or in the width direction of the feeding direction of receiving media P is introduced to the periphery of the polygon motor21, is then introduced to the control unit4and the power source unit5, and, after that, is discharged outside of the apparatus, thereby restricting the increase in temperature of the polygon motor21, preventing the deterioration of image quality and increasing the life of the polygon motor21.

Hereinafter, the developing means20will be described in detail, taking the image forming station Y inFIG. 2as an example. In this embodiment, since the image forming stations Y, M, C, K are arranged obliquely and the image carriers17are in contact with the belt face14a, of which traveling direction is downward, of the intermediate transfer belt14, toner containers26are arranged obliquely downward. For this, special structure is employed in the developing means20.

That is, the developing means20each comprises the toner container26, a toner storage area27formed in the toner container26for storing toner (indicated by hatching), a toner agitating member29disposed inside the toner storage area27, a partition30defined in an upper portion of the toner storage area27, a toner supply roller31disposed above the partition30, a flexible blade32attached to the partition30to abut the toner supply roller31, the development roller33arranged to abut both the toner supply roller31and the image carrier17, and a regulating blade34arranged to abut the development roller33.

The image carrier17is rotated in the traveling direction of the intermediate transfer belt14. The development roller33and the supply roller31are rotated in a direction opposite to the rotational direction of the image carrier17as shown by arrow. On the other hand, the agitating member29is rotated in a direction opposite to the rotational direction of the supply roller31. Toner agitated and scooped up by the agitating member29in the toner storage area27is supplied to the toner supply roller31along the upper surface of the partition30. Friction is caused between the toner and the flexible blade32so that mechanical adhesive force and adhesive force by triboelectric charging are created relative to the rough surface of the supply roller31. By these adhesive forces, the toner is supplied to the surface of the development roller33. The toner supplied to the development roller33is regulated into a coating layer having a predetermined thickness by the regulating blade34. The toner layer as a thin layer is carried to the image carrier17so as to develop a latent image on the image carrier17at and near a nip portion which is a contact portion between the development roller33and the image carrier17.

In this embodiment, the development roller33disposed facing the image carrier17, the toner supply roller31, and the contact portion of the regulating blade34relative to the development roller33are not submerged in the toner in the toner storage area27. This arrangement can prevent the contact pressure of the regulating blade34relative to the development roller33from being varied due to the decrease of the stored toner. In addition, since excess toner scraped from the development roller33by the regulating blade34spills onto the toner storage area27, thereby preventing filming of the development roller33.

The contact portion between the development roller33and the regulating blade34is positioned below the contact portion between the supply roller31and the development roller33. There is a passage for returning excess toner, which was supplied to the development roller33by the supply roller31but not transmitted to the development roller33, and excess toner, which was removed from the development roller33by the regulating operation of the regulating blade34, to the toner storage area27at the lower portion of the developing means. The toner returned to the toner storage area27is agitated with toner in the toner storage area27by the agitating member29, and is supplied to a toner inlet near the supply roller31again. Therefore, the excess toner is let down to the lower portion without clogging the friction portion between the supply roller31and the development roller33and the contact portion between the development roller33and the regulating blade34and is then agitated with toner in the toner storage area27, whereby the toner in the developing means deteriorates slowly so that portentous changes in image quality just after the replacement of the developing means is prevented.

As shown inFIG. 1again, the paper feeding unit10comprises a sheet cassette35in which a pile of receiving media P are held, and a pick-up roller36for feeding the receiving media P from the sheet cassette35one by one.

The paper handling unit11comprises a pair of gate rollers37(one of which is attached to the housing2) for regulating the feeding of a receiving medium P to the secondary transfer portion at the right time, the secondary transfer roller39as a secondary transfer means abutting and pressed against the driving roller12and the intermediate transfer belt14, a main feeding passage38, the fixing means40, a pair of outfeed rollers41, and a dual-side printing passage42.

The fixing means40comprises a pair of fixing rollers40aat least one of which has a built-in heating element such as a halogen heater and which are freely rotatable, and a pressing means for pressing at least one of the rollers against the other roller to fix a secondary image secondarily transferred to the receiving medium P. The secondary image secondarily transferred to the receiving medium is fixed to the receiving medium at the nip portion formed between the fixing rollers40aat a predetermined temperature. In this embodiment, the fixing means40can be arranged in a space formed obliquely above the belt face14b, of which traveling direction is upward, of the transfer belt, that is, a space formed on the opposite side of the image forming stations relative to the transfer belt. This arrangement enables the reduction in heat transfer to the exposure means6, the intermediate transfer belt14, and the image forming means and lessens the frequency of taking the action for correcting color registration error. In particular, the exposure means6is positioned farthest from the fixing means40, thereby minimizing the deformation of the scanning optical components due to heat and thus preventing the occurrence of color registration error.

In this embodiment, since the intermediate transfer belt14is disposed in a slanting direction relative to the driving roller12, a large space is created on the right side of the intermediate transfer belt14in FIG.1. The fixing means40can be disposed in the space, thereby achieving the reduction in size of the apparatus. This arrangement also prevents the heat generated by the fixing means40from being transferred to the exposure unit6, the intermediate transfer belt14, and the respective image forming stations Y, M, C, K which are located on the left side of the fixing means40. Since the exposure unit6can be located in a space on the lower left side of the image forming unit7, the vibration of the scanning optical system due to vibration of the driving system of the image forming means affecting the housing2can be minimized and the deterioration of image quality can be prevented.

Further, in this embodiment, by employing spheroidized toner, the primary transfer efficiency is increased (approximately 100%). Therefore, no cleaning means for collecting residual toner after the primary transfer is used for the respective image carriers17. Accordingly, the image carriers17of which diameter is 30 mm or less can be arranged closely to each other, thereby reducing the size of the apparatus.

Because no cleaning device is used, the corona charging means19is employed as a charging means. When the charging means is a roller, residual toner after the primary transfer on the image carrier17(the amount of which should be small) is deposited on the roller, leading to insufficient charging. On the other hand, since the corona charging means19is a non-contact charging means, toner hardly adheres to the image carriers, thereby preventing the occurrence of insufficient charging.

The actions of the image forming apparatus as a whole will be summarized as follows:

(1) As a printing command (image forming signal) is inputted into the control unit4of the image forming apparatus1from a host computer (personal computer) (not shown) or the like, the image carriers17and the respective rollers of the developing means20of the respective image forming stations Y, M, C, K, and the intermediate transfer belt14are driven to rotate.

(2) The outer surfaces of the image carriers17are uniformly charged by the charging means19.

(3) In the respective image forming stations Y, M, C, K, the uniformly charged outer surfaces of the image carriers17are exposed to selective light corresponding to image information for respective colors by the exposure unit6, thereby forming electrostatic latent images for the respective colors.

(4) The electrostatic latent images formed on the image carriers17are developed by the developing means20to form toner images.

(5) The primary transfer voltage of the polarity opposite to the polarity of the toner is applied to the primary transfer members16of the intermediate transfer belt14, thereby transferring the toner images formed on the image carriers17onto the intermediate transfer belt14sequentially. According to the movement of the intermediate transfer belt14, the toner images are superposed on the intermediate transfer belt14.

(6) In synchronization with the movement of the intermediate transfer belt14on which primary images are transferred, a receiving medium P accommodated in the sheet cassette35is fed to the secondary transfer roller39through the pair of resist rollers37.

(7) The primary-transferred image meets with the receiving medium at the secondary transfer portion. A bias of the polarity opposite to the polarity of the primary transfer image is applied by the secondary transfer roller39which is pressed against the driving roller12for the intermediate transfer belt14by a pressing mechanism (not shown), whereby the primary-transferred image is secondarily transferred to the receiving medium fed in the synchronization manner.

(8) Residual toner after the secondary transfer is carried toward the driven roller13and is scraped by the cleaning means15disposed opposite to the roller13so as to refresh the intermediate transfer belt14to allow the above cycle to be repeated.

(9) The receiving medium passes through the fixing means40whereby the toner image on the receiving medium is fixed. After that, the receiving medium is carried toward a predetermined position (toward the outfeed tray2ain case of single-side printing, or toward the dual-side printing passage42in case of dual-side printing).

FIG. 3is a schematic sectional view showing the entire structure of another embodiment of the image forming apparatus to which the present invention is adopted. The same components as those of the embodiment ofFIG. 1are marked with the same numerals, so description of such components will be omitted. This embodiment is of a type employing the paper delivery method as mentioned above. In this embodiment, therefore, a paper delivery belt43is employed instead of the intermediate transfer belt14.

In this embodiment, a transfer belt unit9and a paper handling unit11are arranged in a door body3. The transfer belt unit9comprises a driving roller12which is disposed in an upper portion of a housing2and is driven by a driving means (not shown) to rotate, a driven roller13and a backup roller44which are disposed diagonally below the driving roller12, a paper delivery belt43which is laid around the three rollers with certain tension and is driven to circulate in a direction indicated by an arrow (the clockwise direction), and a cleaning means15which is in contact with the surface of the paper delivery belt43to oppose the back-up roller44. The driving roller12and the paper delivery belt43are arranged obliquely to the upper left of the driving roller13. Accordingly, a belt tension side (a side tensioned by the driving roller12)43aat the time of driving the paper delivery belt43is on the lower side and a belt slack side43bis on the upper side.

On the back of the paper delivery belt43, transfer members45composed of leaf spring electrodes are disposed. The transfer members45are pressed into contact with the back of the paper delivery belt43by their elastic force at locations corresponding to image carriers17of respective image forming stations Y, M, C, and K. A transfer bias is applied to each transfer member45. The image carriers17of the image forming stations Y, M, C, K are in contact with the belt tension side43aof the paper delivery belt43. As a result of this, the image forming stations Y, M, C, K are arranged to be inclined in an obliquely leftward direction relative to the driven roller13in FIG.3.

Hereinafter, a transfer belt unit according to the present invention will be described.FIG. 4is an exploded perspective view showing an embodiment of the transfer belt unit,FIG. 5is a perspective view partially showing the transfer belt unit shown inFIG. 4in the assembled state,FIG. 6is a perspective view generally showing the transfer belt unit shown inFIG. 4in the assembled state, and FIGS.7(A),7(B) are partial side views for explaining the actions.

InFIG. 4, a tension roller13and a driving roller12are supported by frames50such that these rollers are freely rotatable. A transfer belt14is laid around the driving roller12and the tension roller13with certain tension. Though only one of the frames50is shown in the drawings, the left and right frames are the same in structure.

Each of the frames50is provided with a fitting flange52which is formed by bending an edge portion thereof toward the belt tension side14ato extend along the belt tensioning direction. The frame50has an elongate hole53, extending in the belt tensioning direction, through which a stationary shaft13asupporting the tension roller13is inserted so that the tension roller13is movable because the sliding of the stationary shaft13ais allowed within the elongated hole53.

The frame50is provided with a stub pin54located obliquely below the elongate hole53and further a stub pin55located obliquely below the stub pin54. A turning lever56is rotatably fitted to the stub pin54. On end of the turning lever56is connected to one end of a spring57. The other end of the spring57is fixed to the stub pin55. Therefore, the spring57pulls the one end of the turning lever56in a direction of an arrow inFIG. 5so that the other end of the turning lever56is moved in a direction of another arrow in FIG.5. In the present invention, the elongate hole53, the turning lever56, and the spring57are defined together as a tensioning mechanism for the tension roller13.

A cleaning means15is disposed on the belt tension side14aand has a cylindrical cleaner casing59. Fixed to the outer periphery of the cleaner casing59is a supporting member58which supports a cleaning blade15aas shown in FIG.2. Disposed inside the cleaner casing59is a toner carrying member15bcomposed of a rotational auger for carrying collected toner. The cleaning blade15ais in contact with the transfer belt14at a position where the transfer belt14is wrapped around the tension roller13.

On the outer periphery of the cleaner casing59, a reinforcing wall60is disposed to extending in the axial direction. Disposed on both ends of the reinforcing wall60are supporting walls61extending in the circumferential direction. Each supporting wall61has a receiving groove62extending in the belt tensioning direction. By fitting the receiving groove62to the fitting flange52, the cleaner casing59is positioned in the belt tensioning direction along the fitting flange52. Accordingly, as shown in FIG.7(A), the guide face of the fitting flange52receives the rotational torque acting on the cleaning blade15awhen the transfer belt is rotated, thereby preventing the rotation of the cleaning blade. Therefore, the fitting flanges52of the flames50and the receiving grooves62of the supporting walls61cooperate to function as a rotation preventing member of the cleaning means15.

On each frame50, a position restricting member63made of a plate is fitted and fixed to the cleaner casing59and the stationary shaft13aof the tension roller13. The position restricting member63is provided with through holes64,65for receiving the cleaner casing59and the stationary shaft13a. The position restricting member63is secured by a securing member66composed of a collar to be fixed to the stationary shaft13a. The securing member66has a tapped hole formed in the outer periphery and is fixed to the stationary shaft13aby a screw67.

The securing member66has a flat contact face69formed in the outer periphery thereof. The other end of the turning lever56is in contact with the contact face69, whereby the turning lever56presses the stationary shaft13ato slide along the elongate hole53in the direction of arrow inFIG. 5by the biasing force of the spring57so as to tension the transfer belt14. As shown in FIG.7(B), even when the stationary shaft13adisplaces, the receiving groove62displaces along the fitting flange52and the cleaner casing59is still positioned by the position restricting member63. Therefore, the position of the cleaning blade15arelative to the stationary shaft13a, i.e. the tension roller13is not varied.

Fixed to the frame50is a toner collecting container70which has a shape extending along the side surface of the frame50as shown in FIG.6. The end of the cleaner casing59is connected to an upper portion of the toner collecting container60. Collected toner carried by the carrying member15bfalls down into the toner collecting container70because of gravity.

As apparent from the above description, according to this embodiment, a mechanism for tensioning the belt is arranged on the side surfaces of the transfer belt unit, thereby reducing the size of the transfer belt unit in the belt tensioning direction and thus achieving the reduction in size and cost.

Even when the position of the tension roller is varied due to allowable error in length of the transfer belt and allowable variance in tension, the cleaning means is linked with tension roller to follow such variation. Therefore, the contact position of the cleaning blade relative to the tension roller is not varied, thus preventing the fluctuation of cleaning performance.

Though the present invention is adopted to the apparatus employing the intermediate transfer method in the above embodiment, the present invention may be adopted to an apparatus employing the paper delivery method. Though the tension roller13is a driven roller in the above embodiment, the tension roller13may be a driving roller. In addition, though the transfer belt is laid around the two rollers in the above embodiment, the transfer belt may be laid around three rollers.

Hereinafter, description will now be made as regard to a meandering prevention device for the transfer belt in order to achieve the second object of the present invention.FIG. 8is a sectional view showing one embodiment thereof.

A driven roller13comprises a stationary shaft150made of a metal, a pair of retaining flanges151,152rotatably attached to the stationary shaft150, and a cylindrical roller body153made of aluminum fixed between the retaining flanges151,152. The stationary shaft150penetrates the left and right frames154,154of the transfer belt unit and is fixed by securing members66.

The retaining flanges151,152function as bearing members for supporting the roller body153. One retaining flange151is made of a non-conductive and low sliding resistance resin (e.g. polyacetal), the other retaining flange152is made of a conductive and low sliding resistance resin, and the stationary shaft150is grounded. Therefore, charge remaining on the belt and charge produced by friction between the back of the belt and the surface of the roller can be easily removed. The one flange151is made of non-conductive material and the other flange152having smaller volume is made of conductive material to function as a conductive bearing, thereby minimizing the consumed amount of the conductive material which is expensive.

The retaining flanges151,152comprise shaft portions151a,152a, large-diameter roller fixing portions151b,152bcontinuously formed with the shaft portions151a,152a, and flange portions151c,152cradially projecting from end portions of the outer peripheries of the roller fixing portions151b,152b, respectively. The both ends of the roller body153are fitted around the roller fixing portions151b,152band fixed by the roller fixing portions151b,152band the flange portions151c,152c, respectively.

The flange portion151cof the one retaining flange151further projects radially from the outer surface of the roller body153, while the flange portion152cof the other retaining flange152is flush with the outer surface of the roller body153. The shaft portions151a,152aof the retaining flanges151,152are disposed rotatably relative to the left and right frames via shims156,156.

The other retaining flange152has a ring-like concave portion152dformed in the roller fixing portion152bat a location around the shaft portion152a. Fitted into the concave portion152dis a slidable flange157. The slidable flange157comprises a cylindrical convex portion157ato be fitted into the concave portion152d, a through hole157bformed in the convex portion157a, and a flange portion157cformed at the outer end of the convex portion157to extend radially. The shaft portion152ais inserted through the through hole157bof the convex portion157a, thereby fitting the convex portion157ainto the concave portion152d. A spring159is arranged such that one end of the spring159is in contact with an inner surface of the convex portion157aand the other end of the spring159is fixed to the shaft portion152aby a spring washer160and a snap ring161. Therefore, the spring159is partially disposed inside the retaining flange152, thereby shortening the span between the frames154,154and thus shortening the size of the apparatus in the width direction. This means that the apparatus can be designed compact.

A transfer belt162(the intermediate transfer belt14ofFIG. 1or the paper delivery belt43ofFIG. 3) is wrapped around the outer periphery of the roller body153. One side edge of the transfer belt162is brought in contact with the flange portion151cof the retaining flange151and the other side edge of the transfer belt is brought in contact with the flange portion157cof the slidable flange157. As the transfer belt162is about to meander in this state, the slidable flange157moves within the concave portion152dbecause of the biasing force of the spring159so as to restrain the other side edge of the transfer belt162to keep the one side edge of the transfer belt162in contact with the flange portion151cof the retaining flange151, thereby preventing the meandering of the transfer belt.

This embodiment employs a method for preventing the meandering of the belt by holding the left and right side edges of the belt with the flange portion151cof the retaining flange151and the flange portion157cof the slidable flange157from both sides. The following two matters are important for carrying out this method.

(1) To make the frictional resistance between the back of the belt162and the surface of the roller body153low.

For positioning the side edge of the belt at the flange portion151cof the retaining flange151by biasing force of the slidable flange157, it is required to smoothly propagate the biasing force of the slidable flange157to the retaining flange151side. If the frictional resistance between the back of the belt162and the surface of the roller body153is high, it is difficult to smoothly propagate the biasing force of the slidable flange157toward the retaining flange151side. If the biasing force toward the flange portion151cof the retaining flange151is increased excessively, wrinkle may be created on the belt in a direction (the radial direction of the roller) perpendicular to the direction of the biasing force so as to not only fail to prevent the meandering but also damage the belt such as cracking the edge of the belt. For this, the material having low frictional resistance is used for the whole belt or the back of the belt, and/or an outer layer having low frictional resistance is formed on the surface of the roller.

(2) To design the belt capable of resisting deformation or damage against the force biasing the belt toward the retaining flange151.

The strength of the belt is improved against the force biasing the belt toward the retaining flange151by setting the amount of a portion of the belt to be wrapped around the roller having the meandering prevention mechanism larger than that of the belt to be wrapped around the other roller such that the portion of the belt to be restrained by the meandering prevention mechanism has a half-cylindrical shape. Since the side edges of the belt repeatedly collide with the flange portions151c,157c, respectively so that the side edges easily suffer some cracks, the belt is formed to be thick only along the side edges thereof or reinforced by separate members. Though using a high rigid member for making the belt (making the belt of a material having higher rigidity or inserting a high rigid material such as a metal between belt materials) is also considered, it is not preferable because tension to be applied to the belt for laying the belt in a predetermined configuration should be large so as to increase the driving load and, in addition, when the belt is stopped for a long period of time, a warp (creep) is created at the portion wrapped around the roller.

Now, description will be made as regard to the method of manufacturing the belt for obtaining the aforementioned characteristics (1), (2). As the method of manufacturing seamless belts, there are extrusion moulding method and a centrifugal moulding method. Devised points in the respective methods will be explained below.

FIG. 9is an illustration for explaining a process of manufacturing a belt by the extrusion moulding method. A thermoplastic resin is molded by extruding it with a ring die170into a cylindrical shape. After the moulding, the extruded article is cut to have a desired width W. In this manner, the belt162is manufactured. According to this extrusion moulding, minute groove-like extrusion traces162aare formed in the inner surface of the belt along the width direction of the same. On the other hand, the roller body153shown inFIG. 8has machining traces (cutting traces or grinding traces) formed along the circumferential direction of the roller.

Therefore, since the extrusion traces162aextend perpendicularly to the machining traces when the belt162is laid around the roller body153, the extrusion traces162aand the machining traces do not interfere with each other. This means, the propagation of the biasing force of the slidable flange157is not intercepted, thereby reducing the frictional resistance between the back of the belt162and the surface of the roller body153.

FIGS.10(A)-10(D) are illustrations for explaining a process of manufacturing a belt by a centrifugal moulding method, wherein FIG.10(A) is a sectional view generally showing the belt with a mold, FIG.10(B) is an enlarged sectional view of a portion B of FIG.10(A), FIG.10(C) is a sectional view of the belt manufactured, and FIG.10(D) is a sectional view showing a state that the belt is wrapped around a roller body.

As shown in FIG.10(A), this method is a method of manufacturing a belt162by pouring a liquid resin or liquid rubber material into a cylindrical mold171which rotates. Unlike the extrusion moulding, a multi-layered belt can be manufactured by sequentially pouring materials such as resin, rubber, and the like having different properties. In this embodiment, as shown in FIG.10(B), the belt having three layers is made by sequentially pouring a fluorine content resin162bfor ensuring the cleaning property of the belt surface, an urethane rubber162cas a core material, and a fluorine content resin162dmaking the back of the belt to have lower frictional resistance. At the both side ends of the cylindrical mold171, large-diameter portions171aof which diameter is larger than that of the middle portion are formed and tapered portions171bfor allowing smooth flowing of material into the large-diameter portions171aare formed, thereby facilitating the manufacture of a belt which has side edges thicker than the middle portion. By cutting both sides of the belt manufactured by the centrifugal moulding method as shown in FIG.10(C), the belt, which can be fitted to the roller body153as shown in FIG.10(D), is obtained.

If the side edges of the belt are formed to be too thick, a warp as mentioned above may be created and crack may be created when driven for a long period of time. Accordingly, the thickness at the side edges of the belt should be twice or less of that at the middle portion of the belt. For example, when the middle portion of the belt is 200 μm in thickness, the side ends of the belt are 400 μm or less in thickness.

As apparent from the above description, a meandering prevention mechanism according to this embodiment comprises a roller body around which a transfer belt is wrapped, a pair of retaining flanges arranged on the both ends of the roller body, a flange portion which is formed on one of the retaining flange to project from the outer periphery of the roller body, a slidable flange which is slidably disposed on the other retaining flange, a spring for biasing the slidable flange against the other retaining flange, and a flange portion which is formed on said slidable flange to project from the outer periphery of the roller body. The side edges of the transfer belt are restrained between the flange portion of the one retaining flange and the flange portion of the slidable flange, thereby preventing the meandering of the belt with a simple structure.

Though the present invention has been described with reference to the embodiments disclosed herein, the present invention is not limited thereto and the components of the present invention may be replaced with or include conventionally known or well known techniques.

For example, though the meandering prevention device is arranged in the driven roller13shown in FIG.1andFIG. 3in the above embodiment, the meandering prevention device may be arranged in the driving roller12and may be arranged in both the rollers. Alternatively, a plurality of driven rollers may be provided and the meandering prevention devices may be arranged in the respective driven rollers. In case of the image forming apparatus shown in FIG.1andFIG. 3in which the diameter of the driven roller13is larger than the diameter of the driving roller12, the meandering prevention mechanism is preferably arranged in the driven roller13because the restration is conducted by the retaining flange151and the slidable flange157of which diameters are larger than those of the driving roller12so as to further effectively prevent the meandering.