Image forming apparatus

An image forming apparatus includes a belt unit having a driving roller, a driven roller, a driving roller gear; and a belt which is looped around the driving roller and the driven roller; a belt driving gear which is engaged with the driving roller gear; a photosensitive drum which contacts the belt; and a positioning section which contacts a portion of the belt unit to position the belt unit with respect to a moving direction of the belt. The positioning section contacts the portion of the belt unit in a direction of receiving a force with which the photosensitive drum is pressing the belt unit. The belt driving gear is engaged with the driving roller gear such that a force pressing the belt unit from the belt driving gear via the driving roller gear presses the belt unit against the positioning section.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese Patent Application No. 2009-224028, filed on Sep. 29, 2009, the entire subject matter of which is incorporated herein by reference.

TECHNICAL FIELD

Aspects of the present invention relate to an image forming apparatus.

BACKGROUND

An electrophotographic image forming apparatus uses a belt unit to transport a recording sheet. The belt unit is provided to oppose a photosensitive drum, wherein a toner image is transferred from the photosensitive drum to the recording sheet transported by the belt unit. In addition, when an intermediate transfer belt is used in a belt unit temporarily holding the toner image to be formed on the recording sheet, the belt unit is also provided to oppose a photosensitive drum.

Then, these belt units have to be accurately positioned with respect to an apparatus body in order to form a toner image at an accurate position with respect to a recording sheet and, in case of color printing, in order to prevent occurrence of color shift. For example, in an image forming apparatus, the position of the belt in its moving direction is defined by pressing the bearing of a roller, around which the belt is looped, against a positioning section of a frame.

However, in the image forming apparatus, due to the setting of the operating timing or the peripheral speed of the belt or the photosensitive drum, a force acts to move the belt from the photosensitive drum toward the belt unit. In this image forming apparatus, since this force acts in a direction of separating the bearing of the belt unit away from the positioning section, there is a possibility that the position of the belt in the moving direction of the belt would become unstable.

SUMMARY

Accordingly, it is an aspect of the present invention to provide an image forming apparatus which allows the position of the belt unit with respect to a moving direction of a belt to be stable.

According to an illustrative embodiment of the present invention, there is provided an image forming apparatus comprising: a belt unit including a driving roller, a driven roller, a driving roller gear which is coaxial with the driving roller, and a belt which is looped around the driving roller and the driven roller; a belt driving gear which is engaged with the driving roller gear to transmit a driving force to the driving roller; a photosensitive drum which is configured to rotate while contacting the belt; and a positioning section which is provided to an apparatus body and configured to contact a portion of the belt unit to position the belt unit with respect to a moving direction of the belt. The positioning section is configured to contact the portion of the belt unit in a direction of receiving a force with which the photosensitive drum is pressing the belt unit. The belt driving gear is engaged with the driving roller gear such that a force pressing the belt unit from the belt driving gear via the driving roller gear presses the belt unit against the positioning section.

According to another illustrative embodiment of the present invention, there is provided an image forming apparatus comprising: an apparatus body; a belt unit including a driving roller, a driven roller, a driving roller gear which is coaxial with the driving roller, and a belt which is looped around the driving roller and the driven roller; a belt driving gear which is provided to the apparatus body and is engaged with the driving roller gear; a photosensitive drum which is configured to rotate while contacting the belt; and an abutting section which is provided to the apparatus body at a position configured to contact a portion of the belt unit from a direction opposite to a rotating direction of the photosensitive drum at a contacting point with the belt. The belt driving gear is engaged with the driving roller gear such that an angle formed between a direction extending form a center of the belt driving gear to a center of the driving roller gear and a moving direction of the belt at a contacting point with the photosensitive drum is larger than a pressure angle of the belt driving gear.

DETAILED DESCRIPTION

Illustrative embodiments of the present invention will be described with reference to the accompanying drawings. A color printer1is shown inFIG. 1as an example of an image forming apparatus according to an illustrative embodiment of the present invention. For ease of discussion, in the following description, directions are defined as viewed from a user who operates the printer1. The top or upper side, the bottom or lower side, the left or left side, the right or right side, the front or front side, and the rear or rear side of the printer1are identified as indicated by the arrows in drawings. Further, herein the left-right direction is also referred to as a width direction, and the upper-lower direction is also referred to as a vertical direction. The left-right direction and the front-rear direction are also referred to as a horizontal direction. With regard to various individual components of the printer1, sides of the individual components are similarly identified based on the arranged/attached position of the components on/in the printer1.

Overall Configuration of Color Printer

As shown inFIG. 1, the color printer1includes a body housing10constituting a portion of an apparatus body, a sheet feeding section2which feeds a sheet P, an image forming section3which forms an image on the fed sheet P, and a sheet discharging section4which discharges the sheet P on which the image has been formed.

The sheet feeding section2is provided at a lower part in the body housing10, and includes a sheet tray21in which sheets P are accommodated and a sheet feeding mechanism22which feeds sheet P from the sheet tray21to the image forming section3. Sheets P in the sheet tray21are separated one by one and fed to the image forming section3by the sheet feeding mechanism22.

The image forming section3includes an exposure unit30, an image forming unit40, a belt unit50, a belt cleaner60and a fixing unit70.

The exposure unit30is provided at an upper part in the body housing10and includes a laser light emitting section, a polygon mirror, lenses, a reflective mirror and the like (not shown). Laser light emitted from the laser light emitting section for each of cyan, magenta, yellow and black colors is reflected by the polygon mirror or the reflective mirror, passes through the lenses, and then are irradiated onto the surface of corresponding photosensitive drum43as a high speed scanning.

The image forming unit40is provided between the sheet feeding section2and the exposure unit30, and includes four process units41and a supporting member42supporting the four process units41in a state in which these process units41are aligned in the front-rear direction.

The process unit41includes a photosensitive drum43, a charger44and a developing cartridge45. The developing cartridge45includes a developing roller46and a supply roller, a layer thickness regulating blade and a toner accommodating section (reference numerals are omitted).

The belt unit50is provided between the sheet feeding section2and the image forming unit40, and includes a driving roller51, a driven roller52, an endless transport belt53which is looped around the driving roller51and the driven roller52, and four transfer rollers54. An outside surface of the transport belt53contacts each of the photosensitive drums43. In an inside of the transport belt53, each of the transfer rollers54is provided to interpose the transport belt53with the corresponding photosensitive drum43. Furthermore, the belt unit50is configured to be removably from the body housing10.

The fixing unit70is provided at a rear side of the image forming unit40and the belt unit50, and includes a heating roller71and a pressing roller72which opposes the heating roller71to press the heating roller71.

In the image forming section3, after the surface of the photosensitive drum43is uniformly charged by the charger44, it is exposed by high speed scanning of the laser light from the exposure unit30, so that an electrostatic latent image is formed on the photosensitive drum43. Toner in the toner accommodating section is supplied to the developing roller46via the supply roller and enters between the developing roller46and the layer thickness regulating blade to be held on the developing roller46as a thin layer of a predetermined thickness.

The toner held on the developing roller46is supplied from the developing roller46to the electrostatic latent image on the photosensitive drum43. Accordingly, the electrostatic latent image is visualized into a toner image on the photosensitive drum43. When the sheet P fed onto the transport belt53is transported between the photosensitive drums43and the transport belt53(the transfer rollers54), the toner image formed on each of the photosensitive drums43is sequentially transferred onto the sheet P in an overlapping manner. Then, when the sheet P is transported between the heating roller71and the pressing roller72, the toner image transferred onto the sheet P is fixed by heating.

The sheet discharging section4includes a sheet discharging path81formed to extend upwards from the fixing unit70and change its direction in a forward direction, and a plurality of transport rollers82which transport the sheet P. The sheet P on which the toner image has been fixed by heating is transported along the sheet discharging path81by the transport rollers82and is discharged onto the sheet discharging tray12provided at an upper surface of the body housing10.

A front side of the body housing10is defined by a front cover11which can be opened or closed, and the image forming unit40and the belt unit50can be removed to outside of the body housing10by opening the front cover11.

Detailed Configuration of the Periphery of Belt Unit

Next, the detailed configuration of the periphery of the belt unit50will be described with respect toFIGS. 2 and 3. The transport belt53is looped around the driving roller51and the driven roller52as described above, and an upper side of the transport belt53(hereinafter, referred to as “an upper belt53A”), contacts each of the photosensitive drums43at an outside (upper side) surface thereof. At a time of printing, the photosensitive drums43contact the transport belt53with the sheet P interposed therebetween.

Herein, in this illustrative embodiment, “a moving direction” of the transport belt53indicates a front-rear direction in which the upper belt53A moves. And, “an advancing direction” of the transport belt53indicates a direction in which the upper belt53A or a lower side of the transport belt53(hereinafter, referred to as “a lower belt53B”) actually advances. These directions are distinguished from each other. That is, in this illustrative embodiment, a position in the moving direction means a position in the front-rear direction, the advancing direction of the upper belt53A means the rearward direction, and the advancing direction of the lower belt53B means the forward direction.

In order to transport the sheet P rearward, the upper belt53A advances toward the rear side and the photosensitive drums43rotate in a clockwise direction inFIG. 2. That is, a portion at which the photosensitive drum43contacts the upper belt53A advances toward the rear side same as the upper belt53A. A circumferential speed of the photosensitive drum43(moving speed of the rotating surface) and that of the transport belt53are set to be substantially same. Precisely, the circumferential speed of the photosensitive drum43may be set to be slightly smaller than that of the transport belt53, or the circumferential speed of the photosensitive drum43may be set to be slightly larger than that of the transport belt53. If the circumferential speed of the photosensitive drum43is larger than that of the transport belt53, a force acts in which the photosensitive drum43transports the belt unit50rearward. This situation may be advantageous as described later.

The driving roller51and the driven roller52are supported to be rotatable by a frame55via bearings51A and52A, respectively. In order to apply an appropriate tension to the transport belt53, the driven roller52is urged by a spring (not shown) to be separated form the driving roller51. That is, the driven roller52is supported by the frame55to be movable slightly in the front-rear direction.

The bearing51A and a shaft51B of the driving roller51protrude to both right and left sides. The apparatus body is provided with a positioning rail92(as an example of a positioning member) at both right and left sides of the transport belt53. The positioning rail92includes a rail section92A (as an example of the guide) which extends horizontally in a front-rear rear direction, and an abutting section92B which extends upward from a rear end of the rail section92A. A front surface of the abutting section92B has a vertically extending plane and is configured to receive a force with which the photosensitive drum43presses the belt unit50, that is, a rearward pressing force. The bearing51A has a length which reaches the positioning rail92.

The frame55includes a lock pin55A at a front side position thereof. The lock pin55A protrudes to right and left outsides. In front of the right and left lock pins55A, a lock member93supported by the apparatus body to be pivotable about a shaft93C which is parallel to the driving roller51. The lock member93includes a first arm93A extending upward and a second arm93B extending downward. The second arm93B is engaged with one end of the spring94. The other end of the spring94is engaged with the apparatus body. The spring94always urges the second arm93B toward the front side, and accordingly, the first arm93A is urged toward the rear side. The first arm93A is provided to contact the lock pin55A of the belt unit50mounted in the apparatus body and always urges the belt unit50toward the rear side. An upper end face93D of the first arm93A has a downward slanted face toward the rear side. When mounting the belt unit50into the apparatus body, the upper end face93D contacts the lock pin55A descending from above to allow the first arm93A to move toward the front side, and the bearing52A is put on a support base95provided to the apparatus body so that the position of the bearing52A in the upper-lower direction is defined.

With such a configuration, when the color printer1is not operating, the lock pin55A is pressed toward the rear side by the first arm93A, the entire belt unit50is pressed toward the rear side, and the bearing51A contacts the abutting section92B of the positioning rail92. Accordingly, the belt unit50is positioned in the moving direction of the transport belt53.

One end (left end) of the shaft51B of the driving roller51is provided with a driving roller gear51C which is coaxial with the drive roller51. The driving roller gear51C is engaged with a belt driving gear91at the rear and upper side of the driving roller gear51C. The belt driving gear91is supported to be rotatable by the apparatus body and receives a force from a motor (not shown) to rotate in a clockwise direction inFIG. 2. Accordingly, the driving roller gear51C engaged with the belt driving gear91rotates in a counterclockwise direction inFIG. 2to rotate the driving roller51and the transport belt53in a counterclockwise direction.

A front end of the belt unit50has a handle57fixed to the frame55. The handle57is a part to be gripped in order to remove the belt unit50from the apparatus body.

Under the belt unit50, in other words, on a side of the belt unit50opposite to the photosensitive drum43, a belt cleaner60is provided. The belt cleaner60includes a cleaning roller61, a collecting roller62, a blade63and a collecting box64.

The cleaning roller61rotates about a shaft parallel to the driving roller51in a counterclockwise direction inFIG. 2which is opposite to the rotating direction of the photosensitive drum43to sliding contact the surface of the lower belt53B. The cleaning roller61sliding contact the lower belt53B so that the cleaning roller61scrapes off the remaining toner or sheet powder on the surface of the transport belt53. The belt unit50further includes a backup roller56provided inside of the transport belt53to interpose the lower belt53B with the cleaning roller61. The backup roller56is configured to apply a contact pressure between the lower belt53B and the cleaning roller61.

The collecting roller62rotates in a clockwise direction inFIG. 2while sliding contacting the cleaning roller61under the cleaning roller61to remove the toner attached to the cleaning roller61.

The blade63is provided so that a tip end thereof sliding contacts the collecting roller62, and scrapes off the toner or the like on the surface of the collecting roller62and drops it into the collecting box64.

The arrangement of the belt driving gear91will be described with reference toFIG. 4. The belt driving gear91is provided to have a relation in which, in the side view, an angle β formed between a direction extending from a center O1of the driving roller gear51C to a center O2of the belt driving gear91and the advancing direction of the upper belt53A (a belt contacting the photosensitive drum43) is larger than a pressure angle α of the belt driving gear91(β>α). In this case, the a force F with which the belt driving gear91presses the driving roller gear51C (that is, the force pressing the belt unit50) presses the bearing51A of the belt unit50against the abutting section92B. Specifically, this force F has a component toward the abutting section92B (left-directing component inFIG. 4) with respect to the front-rear direction. Herein, the pressure angle indicates an angle formed between a radial direction of the of the belt driving gear91and a tangential direction of a tooth surface of the belt driving gear91at a point (a pitch point in general) of the tooth surface.

Next, a mounting operation and a movement of the belt unit50in the above-described printer1will be described.

As shown inFIG. 5A, when the belt unit50is mounted in the apparatus body, the front cover11is opened and, in a state where the image forming apparatus40is removed out of the apparatus body, the belt unit50is inserted into the apparatus body so that the driving roller51side goes at first. At this time, the insertion is performed in a state where the driving roller51on a front side with respect to the handle57is slightly lowered. Then, as shown inFIG. 5B, in a state where the bearing51A of the driving roller51is put on the rail section92A of the positioning rail92, the bearing51A slides on the rail section92A toward the rear side until the bearing51A abuts on the abutting section92B.

After the bearing51A has abutted on the abutting section92B, the handle57is lowered downward. And, as shown inFIG. 5C, the lock pin55A abuts on the upper end face93D of the lock member93and the first arm93A is pressed toward the front side. When the handle57is lowered until the bearing52A abuts on the support base95, the position of the belt unit50in the upper-lower direction is defined. As the spring94pulls the second arm93B of the lock member93toward the front side, the first arm93A pushes the lock pin55A toward the rear side and the bearing51A abuts on the abutting section92B without any gap therebetween, whereby the position of the belt unit50in the front-rear direction is defined.

When the image forming unit40is mounted in the apparatus body and printing starts, the photosensitive drum43, the belt driving gear91and the cleaning roller61are rotated. If the photosensitive drum43starts to move at a timing earlier than the transport belt53or if the peripheral speed of the photosensitive drum43is larger than that of the transport belt53, the photosensitive drum43presses the upper belt53A via the sheet P toward the rear side. Unlike a related art, this force causes the bearing51A to abut on the abutting section92B. Accordingly, with the movement of the photosensitive drum43, the position of the belt unit50in the front-rear direction is securely defined and does not become unstable.

The direction in which the belt driving gear91presses the driving roller gear51C is a rearward and downward oblique direction, and this force presses the bearing51A against the rail section92A and the abutting section92B. Accordingly, even with the movement of the belt driving gear91, the position of the belt unit50in the upper-lower direction and in the front-rear direction becomes stable.

In addition, when the cleaning roller61rotates, the cleaning roller61presses the lower belt53B toward a rear side so that the entire belt unit50is pushed toward a rear side. This force also stabilizes the position of the belt unit50in the front-rear direction by pressing the bearing51A against the abutting section92B.

As described above, according to the color printer1according to the illustrative embodiment, since any of forces applied to the belt unit50by the photosensitive drum43, the belt driving gear91and the cleaning roller61acts to press the bearing51A against the abutting section92B, the position of the belt unit50in the front-rear direction becomes stable, so that a printing position with respect to the sheet P becomes accurate, and the color shift is suppressed. In particular, if the peripheral speed of the photosensitive drum43is larger than that of the transport belt53, the photosensitive drum43always presses the belt unit50, so that the effect of stabilizing the position of the belt unit50in the front-rear direction becomes higher.

In this illustrative embodiment, since the bearing51A of the driving roller51to which a large force is applied by the rotational driving is positioned while abutting on the abutting section92B, the flexing of the belt unit50does not likely to affect the position of the belt unit50and the positional accuracy can be maintained.

In addition, since the rail section92A extending in the front-rear direction is provided, it becomes easy to move the bearing51A to the abutting section92B, so that the mounting operability of the belt unit50to the apparatus body improves.

For example, in the above-described illustrative embodiment, although the positioning of the belt unit50in the front-rear direction is performed by causing the bearing51A of the driving roller51to contact the abutting section92B, the shaft51B may contact the abutting section92B.

In addition, in the above-described illustrative embodiment, in order to apply a tension to the upper belt53A by the force driving the transport belt53, the driving roller51is provided at a rear side and the driven roller52is provided at a front side. However, as shown inFIG. 6, a driving roller151may be provided at a front side and a driven roller152may be provided at a rear side. In this case, it is advantageous that by engaging the belt driving gear91with a driving roller gear151C provided coaxially with the driving roller151at an upper and front side, the force with which the belt driving gear91is pressing the belt unit150via the driving roller gear151C has a component which presses the bearing152A of the driven roller152against the abutting section92B.

In addition, the belt cleaner60is not limited to the configuration using the cleaning roller61as in the above-described illustrative embodiment, and a belt cleaner160may be employed which has a blade161which contacts the lower belt53B.

In the above-described illustrative embodiment, the transport belt53which transports a sheet P is provided as one example of a belt. However, the belt may be an intermediate transfer belt to which a toner image is temporarily transferred from the photosensitive drum43.

In addition, in the above-described illustrative embodiment, the color printer1is shown as an example of an image forming apparatus. However, the inventive concept of the present invention can also be applied to a monochrome printer, a copy machine, a multifunction machine or the like.