Image-forming device preventing adverse effects on image formation and on detection of optical sensor

This application claims priority from Japanese Patent Application No. 2006-222494 filed Aug. 17, 2006. The entire content of its priority application is incorporated herein by reference.

1. Field of the Invention

The present invention relates to an electrophotographic image-forming device.

2. Description of the Related Art

One type of conventional electrophotographic image-forming device, such as that disclosed in Japanese unexamined patent application publication No. 2001-183916, has an intermediate transfer belt interposed between photosensitive drums and primary transfer rollers and between a conveyed sheet of recording paper and a secondary transfer roller. After applying a voltage to the primary transfer rollers disposed opposite the photosensitive drums to transfer developer images from the photosensitive drums onto the intermediate transfer belt, the image-forming device applies a voltage to the secondary transfer roller disposed opposite the recording paper to transfer the developer images carried on the intermediate transfer belt to the recording paper, forming an image on the recording paper.

Since a high voltage (transfer bias) is applied to the primary and secondary transfer rollers during the transfer operation, the belt unit, including the intermediate transfer belt, a drive roller for driving the belt, and the like, has a tendency to retain an electric charge. Therefore, the invention disclosed in Japanese unexamined patent application publication No. 2001-183916 grounds the belt unit through an electric-resistive material, such as varistors or resistors, to neutralize the belt, drive roller, and other members targeted for neutralization.

In recent years, image-forming devices have been provided with an optical sensor near the drive roller, which stabilizes the behavior of the belt. The image-forming device reads the density and the like of a resist pattern formed on the belt or developer deposited on the belt and controls operations based on this data.

However, since the drive roller is affected by the transfer bias and carries a high voltage charge, there is a high possibility that the optical sensor disposed near the drive roller will be greatly affected by the high voltage carried on the drive roller. Since the optical sensor cannot accurately detect data when strongly affected by such a high-voltage charge, the data detected by the optical sensor may adversely affect the control of the image-forming device.

Although the charge retained by the belt unit can be quickly removed by grounding the belt unit directly rather than through an electric-resistive material, it is difficult to transfer developer images with stability when the charge is removed too quickly from the belt, running a high risk of not being able to form images with stability.

An image-forming device having a direct tandem system for transferring developer images formed on photosensitive drums directly to a sheet of recording paper rather than through an intermediate transfer belt also has transfer rollers disposed on the side of the conveying belt opposite the photosensitive drums. Hence, an optical sensor disposed near the drive roller driving the conveying belt in such a direct tandem type image-forming device is also greatly affected by the high-voltage charge in the drive roller, and cannot accurately detect data.

In view of the foregoing, it is an object of the present invention to provide an image-forming device capable of preventing adverse effects on image formation, while preventing adverse effects on an optical sensor disposed near the drive roller.

The above and other objects will be attained by an image-forming device that includes a housing, an image-carrying member, a conveying unit, a transferring unit, an optical sensor, a ground connector, a first connection member and a second connection member. The image-carrying member carries a developer image. The conveying unit includes a driving roller rotatable about its rotation shaft, a tension roller disposed in spaced-apart relation with the driving roller, and an endless belt. The endless belt is supported between the driving roller and the tension roller and moved circularly by the driving roller. The endless belt has an area in confrontation with the image-carrying member and conveys a recording sheet placed on the endless belt. The transferring unit is disposed in confrontation with the image-carrying member with the area of the endless belt interposed therebetween. The transferring unit is configured to transfer the developer image on the image-carrying member to the endless belt in a first mode and to the recording sheet placed on the endless belt in a second mode. The optical sensor is disposed near the driving roller and operable in the first mode to detect the developer image on the endless belt. The ground connector is electrically grounded. The first connection member has a first electrical resistance and is connected between the driving roller and the ground connector to allow electrical charges to flow therebetween. The second connection member has a second electrical resistance and is connected between the endless belt and ground connector to allow electrical charges to flow therebetween. The first electrical resistance is less than the second electrical resistance.

As is clear from the above description, “near the driving roller” is the range from the driving roller in which the optical sensor is adversely affected by a charge retained in the driving roller.

In the construction described above, an electrical charge is quickly removed from the driving roller and more gradually removed from the endless belt, thereby simultaneously preventing adverse effects on image formation and adverse effects on the optical sensor disposed near the driving roller.

The problem described above may also be resolved by an image-forming device including a housing, an image-carrying member, a conveying unit, a transferring unit, an optical sensor, a cleaning unit, a ground connector, a first connection member and a second connection member. The image-carrying member carries a developer image. The conveying unit includes a driving roller rotatable about its rotation shaft, a tension roller disposed in spaced-apart relation with the driving roller, and an endless belt. The endless belt is supported between the driving roller and the tension roller and moved circularly by the driving roller. The endless belt has an area in confrontation with the image-carrying member and conveys a recording sheet placed on the endless belt. The transferring unit is disposed in confrontation with the image-carrying member with the area of the endless belt interposed therebetween. The transferring unit is configured to transfer the developer image on the image-carrying member to the endless belt in a first mode and to the recording sheet placed on the endless belt in a second mode. The optical sensor is disposed near the driving roller and operable in the first mode to detect the developer image on the endless belt. The cleaning unit includes a cleaning roller that removes the developer on the endless belt, and a backup roller in peripheral contact with the cleaning roller. The ground connector is electrically grounded. The first connection member has a first electrical resistance and is connected between the driving roller and the ground connector to allow electrical charges to flow therebetween. The second connection member has a second electrical resistance and is connected between the ground connector and at least one selected from a group consisting of the endless belt, the tension roller, and the backup roller, to allow electrical charges to flow therebetween. The first electrical resistance is less than the second electrical resistance.

In the construction described above, an electrical charge is quickly removed from the driving roller and more gradually removed from at least one selected from a group consisting of the endless belt, the tension roller, and the backup roller, thereby simultaneously preventing adverse effects on image formation and adverse effects on the optical sensor disposed near the driving roller.

In the drawings:

FIG. 1 is a side cross-sectional view showing the structure of a laser printer according to a first embodiment of the present invention;

FIG. 2 is a perspective view of a belt unit according to the first embodiment;

FIG. 3 is an enlarged view of a region B shown in FIG. 2 from a perspective along a direction indicated by an arrow A;

FIG. 4 is a perspective view showing the belt unit according to the first embodiment when a conveying belt has been removed;

FIG. 5 is an enlarged view of a section B shown in FIG. 4 ;

FIG. 6 is an explanatory diagram illustrating a grounding circuit according to the first embodiment;

FIG. 7 is a partial perspective view illustrating the mounted state of the grounding circuit;

FIG. 8 is a perspective view from the bottom of the belt unit according to the preferred embodiment when the conveying belt has been removed;

FIG. 9 is an enlarged view of a section A shown in FIG. 8 ; and

FIG. 10 is an explanatory diagram illustrating a contact structure according to a second embodiment of the present invention.

An electrophotographic image-forming device according to an embodiment of the invention will be described while referring to the accompanying drawings.

In this embodiment, the electrophotographic image-forming device of the invention is applied to a laser printer 1 shown in FIG. 1 . Note that in the following description, the expressions “front,” “rear,” “above,” “below,” and “beneath” are used to define the various parts when the laser printer 1 is disposed in an orientation in which it is intended to be used.

As shown in FIG. 1 , the laser printer 1 includes a substantially box-shaped (cubic) housing 3 . A discharge tray 5 is formed on the top surface of the housing 3 for receiving a recording sheet, such as paper sheet or OHP sheet, discharged out of the housing 3 after images have been printed thereon.

A frame member (not shown) formed of a metal or resin material or the like is disposed inside the housing 3 . A feeding unit 20 , an image-forming unit 10 , a discharge roller 91 , and the like are built into the frame member.

The feeding unit 20 includes a paper tray 21 , a feeding roller 22 , a separating pad 23 , a conveying roller 24 , pinch rollers 25 and 27 , and a registration roller 26 . The paper tray 21 is accommodated in the bottommost section of the housing 3 . The feeding roller 22 is disposed above the front end of the paper tray 21 for supplying sheets of paper loaded in the paper tray 21 to the image-forming unit 10 . The separating pad 23 applies a prescribed conveying resistance to the paper so that the feeding roller 22 feeds one sheet of paper at a time.

The conveying roller 24 conveys sheets of paper supplied from the paper tray 21 to the image-forming unit 10 . The pinch roller 25 is disposed in opposition to the conveying roller 24 for pressing the paper against the conveying roller 24 . The registration roller 26 corrects skew in paper conveyed by the conveying roller 24 and for further conveying the paper toward the image-forming unit 10 , and the pinch roller 27 disposed in opposition to the registration roller 26 . A coil spring (not shown) is provided for pressing the pinch roller 25 against the conveying roller 24 . Similarly, a coil spring (not shown) is provided for pressing the pinch roller 27 against the registration roller 26 .

The image-forming unit 10 includes a scanner unit 60 , a fixing unit 80 , a belt unit 100 , a cleaning unit 120 , and a photosensitive unit 70 including four photosensitive drums 71 .

The scanner unit 60 is disposed in the upper section of the housing 3 for forming an electrostatic latent image on the surface of each of the photosensitive drums 71 . Although not shown in the drawings, the scanner unit 60 includes laser emitting sections, polygon mirrors, fθ lenses, and reflecting mirrors.

Each laser emitting section emits a laser beam based on desired image data. The laser beam is reflected by the polygon mirror, passes through the fθ lens, is reflected by the reflecting mirror, and is reflected downward by the reflecting mirror so as to irradiate the surface of the photosensitive drum 71 , thereby forming an electrostatic latent image thereon.

The photosensitive unit 70 includes a photosensitive-unit casing 75 and four developer cartridges 70 K, 70 Y, 70 M, and 70 C that correspond each color of the developer (Black, Yellow, Magenta, and Cyan). Since the four developer cartridges 70 K, 70 Y, 70 M, and 70 C have the same structure, differing only in the color of developer used, the developer cartridges 70 K, 70 Y, 70 M, and 70 C will be collectively referred to as the developer cartridges 70 .

The developer cartridges 70 are detachably provided in the housing 3 beneath the scanning unit 60 . Each developer cartridge 70 is configured of a casing 75 accommodating a developer-accommodating section 74 .

The developer-accommodating section 74 includes a developer-accommodating chamber 74 A, a supply roller 74 B and a developing roller 74 C for supplying developer to the respective photosensitive drum 71 , and a thickness-regulating blade 74 D. The supply roller 74 B rotates to supply developer from the developer-accommodating chamber 74 A toward the developing roller 74 C. Developer supplied onto the developing roller 74 C is carried on the surface of the developing roller 74 C, while the thickness-regulating blade 74 D regulates the thickness of the developer to a uniform prescribed thickness. Subsequently, the developing roller 74 C supplies the developer to the surface of the photosensitive drum 71 exposed by the scanning unit 60 .

The photosensitive drums 71 function to carry images to be transferred onto an endless conveying belt 103 in a test mode or paper placed on the conveying belt 103 in a print mode. Each photosensitive drum 71 is configured in a cylindrical shape, the outermost layer of which is coated with a positive-charging photosensitive layer formed of polycarbonate.

The charger 72 is disposed in opposition to the photosensitive drum 71 at a position diagonally above and rearward of the same and is separated from the photosensitive drum 71 by a prescribed distance. The charger 72 is a Scorotron charger having a charging wire formed of tungsten. The charging wire generates a corona discharge for charging the surface of the photosensitive drum 71 with a uniform positive polarity.

As the photosensitive drum 71 rotates, the charger 72 charges the surface of the photosensitive drum 71 with a uniform positive polarity. Subsequently, the scanning unit 60 irradiates a laser beam onto the photosensitive drum 71 in a high-speed scan, thereby forming an electrostatic latent image on the surface of the photosensitive drum 71 corresponding to an image to be formed on an area of the conveying belt 103 or paper placed on the conveying belt 103 .

Next, as the developing roller 74 C rotates, the positively charged developer carried on the developing roller 74 C comes into contact with the respective photosensitive drum 71 . At this time, the developer is supplied to the electrostatic latent image formed on the surface of the photosensitive drum 71 , i.e. regions of the uniformly charged photosensitive drum 71 that have been exposed by the laser beam and, therefore, have a lower potential. The supplied developer develops the electrostatic latent image into a visible image according to a reverse development process so that a developer image is carried on the surface of the photosensitive drum 71 .

While described in greater detail below, the belt unit 100 is positioned between the feeding unit 20 and the fixing unit 80 and functions to convey paper. Developer images carried on the surfaces of the photosensitive drums 71 are transferred onto the paper conveyed on the belt unit 100 by a transfer bias applied to transferring rollers 73 (described later), which are provided in the belt unit 100 . After images are transferred onto the conveying belt 103 or paper placed on the conveying belt 103 , the belt unit 100 conveys the paper to the fixing unit 80 .

The fixing unit 80 is disposed on the downstream side of the photosensitive drums 71 with respect to the sheet conveying direction. The fixing unit 80 is for thermally fixing the developer transferred onto the recording sheet, and is detachably mounted in the housing 3 .

Specifically, the fixing unit 80 includes a heat roller 81 and a pressure roller 82 . The heating unit 81 is disposed on the printing surface side of the recording sheet and applies conveying force to the recording sheet while heating developer clinging on the recording sheet. The pressure roller 82 is disposed in confrontation with the heat roller 81 and presses a recording sheet interposed between the pressure roller 82 and the heat roller 81 against the heat roller 81 .

When the paper is conveyed to the fixing unit 80 , the fixing unit 80 generates heat for fixing the transferred developer images to the paper, thereby completing image formation.

Next, the belt unit 100 will be described further in detail.

The belt unit 100 is detachably mounted in the housing 3 . As shown in FIG. 4 , the belt unit 100 includes a driving roller 101 that rotates in conjunction with the operation of the image forming unit 10 , a tension roller 102 rotatably disposed in space-apart relation with the driving roller 101 , a conveying belt 103 (see FIG. 2 ), transferring rollers 73 , a backup roller 124 (see FIG. 8 ), and a belt unit flame 104 . The conveying belt 103 (see FIG. 2 ) is wrapped around the driving roller 101 and the tension roller 102 , i.e., supported between the driving roller 101 and the tension roller 102 .

The belt unit frame 104 is configured of side frames 104 A extending in a direction orthogonal to the axial direction of the driving roller 101 for supporting both ends of rotational shafts 101 A and 102 A of the driving roller 101 and tension roller 102 , respectively; a bridge part 104 B extending parallel to the axis of the driving roller 101 for bridging the side frames 104 A; and a frame cover 104 C for covering the side frames 104 A. The side frames 104 A and bridge part 104 B are integrally molded from a resin having excellent mechanical strength. Further, a groove 104 D (see FIG. 3 ) is formed in the frame cover 104 C concentric with the rotational shaft 101 A of the driving roller 101 .

The tension roller 102 is mounted in the belt unit frame 104 so as to be displaceable in a direction away from the driving roller 101 . Bearings formed of resin are mounted on the shaft ends of the tension roller 102 . When the belt unit 100 is mounted in the housing 3 , the tension roller 102 receives the elastic force of a spring or other elastic means (not shown), pushing the tension roller 102 in the direction away from the driving roller 101 . In this way, the tension roller 102 applies a prescribed tension to the conveying belt 103 . The conveying belt 103 moves circularly along with the rotation of the driving roller 101 for conveying paper toward the fixing unit 80 .

As shown in FIGS. 1 and 4 , a plurality (four in the preferred embodiment) of the transferring rollers 73 are arranged at substantially equal intervals between the driving roller 101 and tension roller 102 . When the belt unit 100 is mounted in the housing 3 , the transferring rollers 73 are disposed in opposition to the photosensitive drums 71 on the opposite side of the conveying belt 103 , as shown in FIG. 1 . In other words, the transferring rollers 73 are disposed in confrontation with the photosensitive drums 71 with the area of the endless belt interposed therebetween.

The transferring rollers 73 rotate in association with the circular movement of the conveying belt 103 . As a sheet of paper passes near the photosensitive drums 71 , the transferring rollers 73 apply a voltage of opposite polarity to that of the photosensitive drums 71 (the charge carried by the developer) to the surface of the paper opposite the printing surface, thereby transferring developer carried on the surfaces of the photosensitive drums 71 to the printing surface of the paper or to the conveying belt 103 .

An optical density sensor 94 is also disposed near the driving roller 101 for detecting the amount (density) of developer deposited on the area of the conveying belt 103 . In the preferred embodiment, the density sensor 94 is disposed in confrontation with the driving roller 101 through the conveying belt 103 . In other words, the density sensor 94 is disposed in confrontation with the conveying belt 103 and detects the density of developer deposited on the conveying belt 103 . More specifically, the density sensor 94 reads a test pattern on the conveying belt 103 for adjusting the density of developer formed on the conveying belt 103 in a test mode. The test mode is a mode in which the transferring rollers 73 transfer developer image to the conveying belt 103 .

As shown in FIGS. 5 and 6 the driving roller 101 has the rotational shaft 101 A formed of a metal, such as stainless steel or iron, and a resin layer 101 B formed of a resin having a high coefficient of friction, such as rubber or urethane, and disposed around the peripheral surface of the rotational shaft 101 A in a region corresponding to the conveying belt 103 (see FIG. 1 ).

The rotational shaft 101 A is rotatably supported in the side frames 104 A via sliding bearings 101 E formed of an electrically conductive resin having a prescribed electrical resistance.

A cap 101 C formed of metal (brass in the preferred embodiment) is provided on one axial end of the rotational shaft 101 A for covering the end portion of the same. The inner peripheral surface of the cap 101 C slidably contacts the outer peripheral surface of the rotational shaft 101 A.

As shown in FIG. 3 , a stopper 101 D protrudes radially outward from the outer peripheral surface of the cap 101 C. The stopper 101 D is engaged in the groove 104 D formed in the frame cover 104 C to prevent the cap 101 C from rotating together with the rotational shaft 101 A so that the cap 101 C is in a fixed state and does not rotate relative to the frame cover 104 C. In the other words, the cap 101 C is held stationary regardless of rotation of the rotational shaft 101 A of the driving roller 101 .

When the belt unit 100 is mounted in the housing 3 , the ground connector 93 in a coiled state contacts the outer periphery of the cap 101 C, as shown in FIG. 6 . In other words, the ground connector 93 has a coil spring section. The ground connector 93 is urged against the cap 101 C with urging force of the coil spring section.

As shown in FIG. 4 , neutralizing plates 105 formed of metal (stainless steel or copper in the preferred embodiment) are disposed between adjacent transferring rollers 73 of the belt unit 100 for removing the charge carried by the conveying belt 103 and the like. The neutralizing plates 105 are disposed between the tension roller 102 and the driving roller 101 such that each neutralizing plate 105 is interposed between adjacent two transferring rollers 73 . As shown in FIG. 7 , each of the neutralizing plates 105 is electrically connected to a connecting wire 106 described later via a connecting wire 105 A. An electrode member (not shown) is also provided on the belt unit 100 extending in the displacement direction of the tension roller 102 for contacting the resin bearings mounted on the shaft ends of the tension roller 102 . The electrode member is electrically connected to the cap 101 C.

As shown in FIG. 1 , the belt cleaner 120 is disposed below the belt unit 100 . The belt cleaner 120 functions to remove developer from the surface of the conveying belt 103 . The belt cleaner 120 is configured of a cleaning roller 121 , a cleaning shaft 122 , a waste developer collecting unit 123 , the backup roller 124 , a scraping blade 125 , and a waste developer feed pump mechanism 126 .

The cleaning roller 121 contacts the conveying belt 103 while rotating in the direction opposite the movement of the conveying belt 103 . In this way, the cleaning roller 121 removes developer deposited on the conveying belt 103 by scraping the developer off the same.

The cleaning shaft 122 rotates while in contact with the outer surface of the cleaning roller 121 . At this time, a voltage of opposite polarity to the charge carried by the developer is applied to the cleaning shaft 122 , transferring the developer from the surface of the cleaning roller 121 to the cleaning shaft 122 , thereby removing the waste developer from the cleaning roller 121 .

The scraping blade 125 is a thin blade configured to scrape off waste developer transferred to the surface of the cleaning shaft 122 . Subsequently, the waste developer feed pump mechanism 126 conveys the waste developer scraped off by the scraping blade 125 to the waste developer-collecting unit 123 .

As shown in FIGS. 6 , 8 , and 9 , the backup roller 124 is rotatably mounted on the belt unit frame 104 through sliding bearings 127 formed of an electrically conductive resin. More specifically, the backup roller 124 has a shaft 124 A that is rotatably supported in bearing parts 127 C of the sliding bearings 127 . Metal compression springs 128 generate an elastic force for urging the sliding bearings 127 toward the cleaning roller 121 . In other words, the backup roller 124 is in peripheral contact with the cleaning roller 123 . The backup roller 124 is electrically connected to the compression spring 128 through the sliding bearings 127 .

As shown in FIG. 9 , each of the sliding bearings 127 has a lever configuration with a first longitudinal end 127 B pivotably coupled with a pivoting shaft 104 E of the belt unit frame 104 , and a second longitudinal end 127 A positioned on the opposite side of the backup roller 124 from the pivoting shaft 104 E. The elastic force of the compression spring 128 is applied to the second longitudinal end 127 A of the sliding bearing 127 to urge the backup roller 124 toward the cleaning roller 121 .

As shown in FIGS. 6 and 7 , one end of the compression spring 128 constitutes part of the connecting wire 106 (connecting wire 106 D) that extends toward the driving roller 101 side. The connecting wire 106 is electrically connected to the backup roller 124 via the sliding bearing 127 .

As shown in FIG. 7 , the connecting wire 106 is configured of a connecting wire 106 C on the driving roller 101 side, the connecting wire 106 D on the backup roller 124 side, and a connecting plate 106 A electrically connecting the connecting wires 106 C and 106 D. More specifically, the connecting wires 106 C and 106 D are fixed to the connecting plate 106 A by metal plus screws 106 B. The connecting wires 106 C and 106 D are formed of metal wire having elasticity, such as a steel spring material.

The driving roller 101 end of the connecting wire 106 C is twisted in a coil shape that contacts and is electrically connected to the outer peripheral surface of the sliding bearing 101 E. The connecting wire 106 C also applies an elastic force to the outer peripheral surface of the sliding bearing 101 E. With this construction, the neutralizing plates 105 , backup roller 124 , and other neutralization targets are electrically connected to the rotational shaft 101 A via the connecting wire 106 and sliding bearing 101 E. As described above, the rotational shaft 101 A is connected to a ground connector 93 via the cap 101 C and grounded by the ground connector 93 . The ground connector 93 is provided on the frame member of the housing 3 . The ground connector 93 is a metal wire having elasticity that is twisted into a coil spring shape, such as a steel spring member, and is grounded.

In the construction described above, the sliding bearing 101 E is electrically connected between the neutralizing plates 105 and backup roller 124 and the cap 101 C having a prescribed electrical resistance, while the rotational shaft 101 A of the driving roller 101 is electrically connected to the cap 101 C through the sliding bearing 101 E. Therefore, the electrical resistance between the driving roller 101 and the cap 101 C is less than that between the neutralizing plates 105 (the conveying belt 103 ) and the cap 101 C. For example, the electrical resistance between the driving roller 101 and the cap 101 C is several ohms, while the electrical resistance between the neutralizing plates 105 (the conveying belt 103 ) and the cap 101 C is several thousand ohms. By the same token, the electrical resistance between the driving roller 101 and the cap 101 C is less than that between the backup roller 124 and the cap 101 C. And, the electrical resistance between the driving roller 101 and the cap 101 C is less than that between the tension roller 102 and the cap 101 C.

Hence, an electrical charge is quickly removed from the driving roller 101 and more gradually removed from the neutralizing plates 105 (the conveying belt 103 ), the tension roller 102 , and the backup roller 124 , thereby simultaneously preventing adverse effects on image formation and adverse effects on the density sensor 94 disposed near the driving roller 101 .

Since the belt unit 100 is detachably mounted in the housing 3 , it is preferable to minimize the number of electrical contacts between the belt unit 100 and the body of the laser printer 1 .

To achieve this, the driving roller 101 and the tension roller 102 , neutralizing plates 105 , and backup roller 124 are all connected to the ground connector 93 in the housing 3 through the cap 101 C. In other words, the cap 101 C is connected between the ground connector 93 and the driving roller 101 , the tension roller 102 , the backup roller, and the neutralizing plates 105 (the conveying belt 103 ). Accordingly, this construction reduces the number of electrical contacts between the belt unit 100 and housing 3 .

Further, although the rotational shaft 101 A of the 101 is formed of metal, the resin layer 101 B is provided on the portion of the rotational shaft 101 A corresponding to the conveying belt 103 . Formed of a resin, such as rubber or urethane, the resin layer 101 B functions as an electric-resistive material.

Hence, when a transfer bias is applied to the transferring rollers 73 , this construction prevents an excessive charge migrating from the driving roller 101 toward the ground connector 93 side via the conveying belt 103 , thereby preventing adverse effects on image formation. However, the charge retained by the metal rotational shaft 101 A of the driving roller 101 migrates quickly to the ground connector 93 via the cap 101 C, thereby suppressing adverse effects on the ground connector 93 .

Further, since the sliding bearing 101 E also functions as a grounding resistance member for the second neutralization targets, it is not necessary to provide a separate electric-resistive material for this purpose, thereby simplifying the structure of the belt unit 100 .

Since the belt unit 100 is detachably mounted in the housing 3 , there is potential for the contact point between the cap 101 C and the ground connector 93 to become misaligned. If the cap 101 C and the ground connector 93 were configured to form a sliding contact, there is great risk that this contact point would be unstable.

To avoid this, a unit-side ground connector electrically connected to the ground connector 93 is configured of the cap 101 C that slidably contacts the outer peripheral surface of the rotational shaft 101 A. Further, since the cap 101 C contacts the ground connector 93 in a fixed state, without rotation or displacement relative to the ground connector 93 , the ground connector 93 and the cap 101 C contact each other in a fixed state rather than a slidable state, thereby stabilizing the contact point between the cap 101 C and the ground connector 93 .

Further, the backup roller 124 is electrically connected to the cap 101 C via the metal compression spring 128 electrically connected to the backup roller 124 , thereby simplifying the structure of the belt unit 100 .

The present invention is not particularly limited to the contact structure described in the preferred embodiment for supplying a transfer bias from the laser printer 1 to the transferring rollers 73 . For example, the following structure shown in FIG. 10 may be employed.

As shown in FIG. 10 , each of the transferring rollers 73 has a metal shaft 73 A rotatably supported in sliding bearings 73 B. Coil springs 73 C urge the sliding bearings 73 B toward the conveying belt 103 (upward toward the photosensitive drum 71 ).

A cap 73 D formed of a conductive resin is mounted over the outer surface of the shaft 73 A on an axial end thereof. The cap 73 D is capable of sliding over the outer peripheral surface of the shaft 73 A to be displaced relative to the shaft 73 A in the axial direction. A coil spring 73 E or other elastic means is provided for urging the cap 73 D toward a contact point 73 F on the body of the belt unit in the axial direction of the shaft 73 A.

A main body transfer bias contact point 95 provided on the main body of the laser printer 1 is urged toward the contact point 73 F for pressing the contact point 73 F against the cap 73 D to connect the two. In this way, a transfer bias can be applied to the transfer roller 73 through the transfer bias contact point 95 .

With this construction, the contact point (cap 73 D) on the transfer roller 73 side reliably contacts the contact point 73 F, even when the elastic force of the coil spring 73 C displaces the transfer roller 73 . Accordingly, a transfer bias can be reliably provided to the transfer roller 73 .

The application of the contact structure described above is not limited to the transfer roller 73 , but may also be applied to other rollers having an electrical contact, such as the driving roller 101 .

Further, while the laser printer 1 according to the preferred embodiment described above has the belt cleaner 120 , the present invention is not limited to this structure and may be applied to an image-forming device without the belt cleaner 120 .

Further, while the laser printer 1 according to the preferred embodiment described above has the neutralizing plates 105 , the present invention is not limited to this construction. For example, the present invention may be applied to an image-forming device without the neutralizing plates 105 or to an image-forming device having neutralizing needles or brush in place of the neutralizing plates 105 .

Further, while the preferred embodiment uses the tension roller 102 , neutralizing plates 105 , and the backup roller 124 as an example of the second neutralization targets, the present invention is not limited to these components.

Further, in the preferred embodiment, the backup roller 124 is electrically connected to the compression spring 128 via the sliding bearing 127 , but the present invention is not limited to this construction. For example, part of the compression spring 128 may be twisted into a coil spring part that is directly connected to the backup roller 124 .

Further, in the preferred embodiment described above, the cap 101 C is provided on an axial end of the rotational shaft 101 A, and the rotational shaft 101 A is electrically connected to the ground connector 93 via the cap 101 C, but the present invention is not limited to this construction. For example, the ground connector 93 may be directly connected to the rotational shaft 101 A, eliminating the cap 101 C.

Further, in the preferred embodiment described above, the unit-side ground connector is provided on an axial end of the rotational shaft 101 A, but the present invention is not limited to this construction.

Further, in the preferred embodiment described above, the stopper 101 D is engaged in the groove 104 D formed in the frame cover 104 C to fix the cap 101 C so that the cap 101 C does not rotate relative to the frame cover 104 C (belt unit frame 104 ) and the like, but the present invention is not limited to this construction. For example, the cap 101 C may be permanently fixed to the frame cover 104 C.

Further, in the preferred embodiment described above, the present invention is applied to an image-forming device having a direct tandem system, but the present invention is not limited to this application and may be applied to an image-forming device having an intermediate transfer belt.