Driving device and image forming apparatus

A driving device includes a stretched member, and a first rotation member and a second rotation member that support the stretched member in a stretched manner. The first rotation member has a first rotation axis, and the second rotation member has a second rotation axis. The first rotation member includes a plurality of members arranged in an axial direction of the first rotation axis.

BACKGROUND OF THE INVENTION

The present invention relates to a driving device in which a stretched member (for example, as an endless belt) is stretched around a plurality of rollers and moved the by the rollers, and an image forming apparatus using the driving device.

There has been proposed a technology for preventing the skew of the endless belt (Japanese Laid-open Patent Publication No. 2006-162659).

However, although the prior art is capable of preventing the skew of the endless belt, a lengthening of a lifetime of the endless belt (i.e., the stretched member) is not sufficiently achieved.

SUMMARY OF THE INVENTION

In an aspect of the present invention, it is intended to provide a driving device and an image forming apparatus capable of lengthen a lifetime of a stretched member.

According to an aspect of the present invention, there is provided a driving device including a stretched member, and a first rotation member and a second rotation member around which the stretched member is stretched. The first rotation member has a first rotation axis, and the second rotation member has a second rotation axis. The first rotation member includes a plurality of members arranged in an axial direction of the first rotation axis.

With such a configuration, a lifetime and reliability of the stretched member can be enhanced.

According to another aspect of the present invention, there is provided an image forming unit including the above described driving device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

First Embodiment

FIG. 1is a schematic view showing a configuration of an image forming apparatus10according to the first embodiment of the present invention.

The image forming apparatus10is configured as, for example, an electrophotographic printer of an intermediate transfer type. The image forming apparatus10includes a medium tray11in which recording media (for example, sheets) P are stored. A medium feeding unit12is provided on a feeding side (i.e., left side inFIG. 1) of the medium tray11. The medium feeding unit12is configured to feed the recording medium P one by one out of the medium tray11. The medium feeding unit12includes a pickup roller12apressed against the topmost recording medium P lifted to a predetermined height. The medium feeding unit12further includes a feeding roller12band a retard roller12cfor separately feeding the recording medium P picked up by the pickup roller12a. A medium conveying unit13is provided on a downstream side of the medium feeding unit12in a conveying direction of the recording medium P. The medium conveying unit13includes a plurality of conveying roller pairs13a,13band13cfor conveying the recording medium P toward a transfer roller15described later.

An image forming portion20includes four toner image forming units30(30C,30M,30Y and30K) as developer image forming units, four transfer rollers14(14C,14M,14Y and14K), and a transfer roller15. The toner image forming units30are arranged in tandem, and respectively form toner images (i.e., developer images). The transfer rollers14are configured to primarily transfer the toner images to an intermediate transfer belt41described later. The transfer roller15is configured to secondarily transfer the toner image from the intermediate transfer belt41to the recording medium P. Therefore, the transfer rollers14are also referred to as primary transfer rollers, and the transfer roller15are also referred to as a secondary transfer roller.

The toner image forming units30include OPC (Organic Photo Conductor) drums31(31C,31M,31Y,31K) as image bearing bodies that bear toner images, charging rollers32(32C,32M,32Y,32K) as charging members that negatively charge the surfaces of the OPC drums31, printing heads33(33C,33M,33Y,33K) as exposure units that expose the surfaces of the OPC drums31to form latent images, developing rollers34(34C,34M,34Y,34K) as developing members that develop the latent images to form toner images, and developer supply units35(35C,35M,35Y and35K) that supply toners to the developing rollers34. The printing heads33are constituted by, for example, LED (Light Emitting Diode) arrays.

A transfer belt unit40as a driving device (i.e., a belt driving device) includes an intermediate transfer belt41(i.e., a stretched member). The intermediate transfer belt41also functions as a toner (developer) image bearing body. The intermediate transfer belt41is an endless belt, and is configured to carry the toner image having been primarily transferred by the transfer rollers14. The transfer belt unit40further includes a driving roller42as a second rotation member, a tension roller43as a first rotation member, and a backup roller44. The driving roller42is driven by a driving motor110, and drives the intermediate transfer belt41in a belt conveying direction shown by an arrow X corresponding to counterclockwise direction inFIG. 1. The tension roller43is provided so as to face the driving roller42. The intermediate transfer belt41is stretched (wound) around the driving roller42, the tension roller43and the transfer roller15. The backup roller44is provided so as to face the transfer roller15via the intermediate transfer belt41.

The transfer belt unit40(as the driving unit) includes a correction portion50(FIG. 10) at an end of the tension roller43. The correction portion50includes an arm52, springs53L and53R, bearings54L and54R, a lever55and a pulley56. Detailed description of these parts will be made later.

A fixing portion16is provided on the downstream side of the transfer roller15(as the secondary transfer roller). The fixing portion16is configured to fix the toner image (i.e., the developer image) to the recording medium P by applying heat and pressure. The fixing portion16includes an upper roller16aand a lower roller16bboth of which have surface layers made of resilient bodies. The upper roller16aand the lower roller16bhave halogen lamps16cand16d(as internal heat sources) therein.

Ejection roller pairs17a,17band17care provided on the downstream side of the fixing portion16. The ejection roller pairs17a,17band17ceject the recording medium P to the outside of the image forming apparatus10. A stacker portion18is provided on an upper part of the image forming apparatus10on which the ejected recording medium P is placed.

The image forming apparatus10has a power source120. The power source120supplies electric power for entire operation of the image forming apparatus10. In particular, the power source120applies voltages to the charging rollers32(32C,32M,32Y,32K), the developing rollers34(34C,34M,34Y,34K), the primary transfer rollers14(14C,14M,14Y,14K) and the secondary transfer roller15.

FIG. 2is a block diagram showing a control system of the image forming apparatus10of the first embodiment.

An image forming control unit100as a control unit includes a microprocessor, ROM, RAM, input-output port, timer and the like. The image forming control unit100receives image data (print data) and control command from a host device10A, and performs sequence control of the entire image forming apparatus10to thereby perform a printing operation.

An I/F control unit101sends printer information to the host device10A, analyzes command sent from the host device10A, and processes data sent from the host device10A.

A charge voltage control unit102controls application of voltages to the charging rollers32to thereby charge the surfaces of OPC drums31according to a command from the image forming control unit100.

A head control unit103controls the printing heads33to emit lights to expose the surfaces of the OPC drums31according to a command from the image forming control unit100so as to form latent images the OPC drums31.

A developing voltage control unit104controls application of voltages to the developing rollers34according to a command from the image forming control unit100so as to cause the toner (i.e., developer) to adhere to the latent images formed on the surfaces of the OPC drums31by the printing heads33.

A primary transfer voltage control unit105controls application of voltages to the (primary) transfer rollers14according to a command from the image forming control unit100so as to transfer the toner images on the surfaces of the OPC drums31to the intermediate transfer belt41(as the endless belt or the developer image bearing body).

A secondary transfer voltage control unit106controls application of a voltage to the secondary transfer roller15according to a command from the image forming control unit100so as to transfer the toner image from the intermediate transfer belt41to the recording medium P.

An image forming driving control unit107controls drive motors112C,112M,112Y,112K for rotating the OPC drums31, the charging rollers32, the developing rollers34according to a command from the image forming control unit100.

A belt driving control unit108controls the driving motor110according to a command from the image forming control unit100so as to rotate the driving roller42to move the intermediate transfer belt41. The rotation of the driving roller42is transmitted to the tension roller43and the backup roller44via the intermediate transfer belt41, and the tension roller43and the backup roller also rotate. The transfer roller15contacting the intermediate transfer belt41also rotates.

A feeding-conveying control unit109controls a feeding motor115and a conveying motor116according to a command from the image forming control unit100so as to feed and convey the recording medium P. In this regard, the feeding motor115drives the pickup roller12a, the feeding roller12b, and the conveying roller pairs13aand13b. The conveying motor116drives the conveying roller pair13c.

A fixing control unit111controls application of voltages to heaters16cand16dof the fixing portion16according to a command from the image forming control unit100so as to fix the toner image to the recording medium P. More specifically, the fixing control unit111receives temperature information from a thermistor113for detecting the temperature of the fixing portion16, and performs ON/OFF control of the heaters16cand16d. Further, the fixing control unit111controls a fixing motor114according to a command from the image forming control unit100so as to rotate the upper and lower rollers16aand16bafter the temperature in the fixing portion16reaches to a predetermined temperature. The fixing motor117drives the upper roller16aof the fixing portion16and the ejection roller pairs17a,17band17c.

FIG. 3is a perspective view showing a basic configuration of the transfer belt unit40according to the first embodiment.FIG. 4is a sectional view taken along line IV-IV inFIG. 3.

The transfer belt unit40is configured so that the intermediate transfer belt41is stretched around three rollers: the driving roller42, the tension roller43and the backup roller43as described above. The driving roller42rotates to move the intermediate transfer belt41e. The tension roller43has a tension roller shaft43awhose inclination can be changed as described later.

FIG. 5shows the driving roller42. As shown inFIG. 5, the driving roller42has a driving roller shaft42b. The driving roller shaft42bis rotatably supported by bearings42L and42R mounted to frames51L and51R (FIG. 3) of the transfer belt unit40. A driving gear42ais fixed to the driving roller shaft42b. A power of the driving motor110is transmitted to the driving gear42a, and the driving roller42(with the driving roller shaft42band the driving gear42a) rotates about a rotation axis O1as a second rotation axis.

Further, the driving roller42is a metal roller made of aluminum covered with a ceramic coating layer. When the driving roller42rotates, the intermediate transfer belt41rotates due to a friction between the driving roller42and the intermediate transfer belt41.

As shown inFIG. 4, the backup roller44is located on a downstream side of the driving roller42in the belt conveying direction X. The backup roller44is made of aluminum, and is rotatably supported by the bearings45L and45R mounted to the frames51L and51R (FIG. 3).

The tension roller43is located on a downstream side of the backup roller44in the belt conveying direction X. The tension roller43has the tension roller shaft43arotatable about a rotation axis O2as a first rotation axis. As shown inFIG. 3, the tension roller43is divided into a plurality of (for example, five) roller parts43-1,43-2,43-3,43-4and43-5in an axial direction of the tension roller shaft43a. That is, the tension roller43as the first rotation member includes a plurality of roller parts43-1,43-2,43-3,43-4and43-5as a plurality of divided rollers (or segment rollers) in the axial direction of the rotation axis O2of the tension roller shaft43a.

FIG. 6is a perspective view showing the roller part43-1among the roller parts43-1through43-5of the tension roller43ofFIG. 3. The roller parts43-1through43-5have engaging holes (i.e., center holes) through which the tension roller shaft43apenetrates.

Therefore, the roller parts43-1through43-5are independently rotatable about the tension roller shaft43a. Further, the roller parts43-1through43-5are mounted to the tension roller shaft43ausing e-rings58so as not to move in the axial direction of the tension roller shaft43a(FIGS. 7A,7B and7C).

As shown inFIG. 7A, a pulley56as a third rotation member is mounted to an end of the tension roller shaft43a. The pulley56has a flange portion56bas a contact portion (i.e., a belt contact portion) with a surface A that contacts a lateral end (i.e., a widthwise end) of the intermediate transfer belt41. The pulley56has a engaging hole56through which the tension roller shaft43apenetrates. The pulley56is slidable along the tension roller shaft43a, i.e., movable in the direction of the rotation axis O2. The pulley56has a surface B opposite to the surface A. The surface B of the pulley56contacts a lever55(as a shaft shifting member). The lever55is mounted to the frame51L so as to be rotatable about a rotation axis O3as a third rotation axis inclined with respect to the rotation axis O2.

A bearing54L is provided on the same end of the tension roller shaft43aas the pulley56. As shown inFIG. 3, an arm52is rotatably mounted to the frame51L so as to be rotatable about a rotation axis52a. The bearing54L is mounted in a rail portion52bformed on the arm52so as to be slidable in a longitudinal direction of the rail portion52b.

A spring53L is provided between the bearing54L and an inner wall of the rail portion52bof the arm52. The spring53L is constituted by a compression coil spring, and presses the bearing54L to apply a tension to the intermediate transfer belt41.

A bearing54R is provided on an end of the tension roller shaft43aopposite to the pulley56. The bearing54R is slidably mounted in a rail portion (not shown) formed on the frame51R. A spring53R (FIG. 4) is provided between the bearing54R and an inner wall of the rail portion of the frame51R (FIG. 2). The spring53R is constituted by a compression coil spring, and presses the bearing54R to apply a tension to the intermediate transfer belt41.

As shown inFIG. 4, a belt regulation roller pair57as a belt regulating unit is provided on a downstream side of the tension roller43in the belt conveying direction X. The belt regulation roller pair57includes rollers57aand57bprovided so as to nip the intermediate transfer belt41therebetween. Both ends of the roller57aare rotatably supported by not shown bearings mounted to the frame51L and51R. Similarly, both ends of the roller57bare rotatably supported by not shown bearings mounted to the frame51L and51R. The rollers57aand57bregulate a trajectory of movement of the intermediate transfer belt41.

The transfer rollers14(14C,14M,14Y,14K) as first primary transfer members are provided on a downstream side of the belt regulation roller pair57in the belt conveying direction X. Each of the transfer rollers14is rotatably supported by not shown bearings mounted to the frames51L and51R. The transfer rollers14are pressed against the OPC drums31C,31M,31Y and31K via the intermediate transfer belt41by a pressing unit (not shown).

As shown inFIG. 7A, an e-ring58and a spacer59are provided between the roller part43-5and the bearing54R. Further, another e-ring58is provided between the roller part43-1and the bearing54L. The e-rings58and the spacer59constitute a regulating member that regulates the axial movement of the roller parts43-1through43-5in the axial direction of the tension roller43. The pulley56has the flange portion56bthat contacts the lateral end of the intermediate transfer belt41as described above. The lever55contacts the surface B of the pulley56opposite to the intermediate transfer belt41. The lever55is mounted to the frame51L so as to be rotatable about the rotation axis O3as the third rotation axis.

The roller parts43-1,43-2,43-3,43-4and43-5of the tension roller43are rotatably supported by the tension roller shaft43a. Gaps “d” are formed between adjacent roller parts43-1through43-5in the axial direction of the rotation axis O2of the tension roller43so as to suppress generation of a friction force.

As shown inFIG. 7B, the gaps “d” are formed by providing ring-shaped boss portions43b(i.e., abutting portions) on the roller parts43-1through43-5. Each boss portion43bhas a smaller diameter than a belt stretching portion43c(of each tension roller43) around which the intermediate transfer belt41is stretched. The boss portions43bof the respective roller parts43-1through43-4abut against to-be-abutted portions43dof the adjacent roller parts43-2through43-5.

In this embodiment, the roller parts43-1through43-5have the same shapes in order to contribute to reducing manufacturing cost. Therefore, the roller parts43-1through43-5have the boss portions43b(i.e., the abutting portions) on the same side, which abut against the to-be-abutted portions43dof the adjacent roller part. However, this embodiment is not limited to such a configuration. For example, it is also possible that each of the roller parts43-2and43-4has two boss portions43bon both sides, and each of the roller parts43-1,43-3and43-5has two to-be-abutted portions43don both sides. With such a configuration, the above described gap “d” can be formed between the adjacent roller parts43-1through43-5, and therefore generation of a friction force can be suppressed.

The tension roller43is supported by engagement of the tension roller shaft43aand the bearings54L and54R. The tension roller43is prevented from moving toward the bearing54R by the e-ring58and the spacer59. Further, the tension roller43is prevented from moving toward the bearing54L by the e-ring58. The bearings54L and54R have self-aligning function, and are configured to follow the inclination of the tension roller43.

In a state shown inFIG. 7B, the rotation axis O2of the tension roller43is parallel to the rotation axis O1of the drive roller42. In this state, the intermediate transfer belt41moves stably.

In a state shown inFIG. 7A, the rotation axis O2of the tension roller43is inclined upward with respect to the rotation axis O1of the drive roller42. In this state, the lever55rotates about the rotation axis O3, and reaches the vicinity of the bearing54L.

In a state shown inFIG. 7C, the rotation axis O2of the tension roller43is inclined downward with respect to the rotation axis O1of the drive roller42. In this state, the lever55rotates about the rotation axis O3to press the pulley56, and reaches a position closer to the bearing54R.

FIG. 8is an enlarged view showing a configuration at the end of the tension roller43on the pulley56side. InFIG. 8, the rotation axis O2of the tension roller43is inclined downward with respect to the rotation axis O1of the driving roller42as shown inFIG. 7C. According to the inclination of the tension roller43, the lever55rotates about the rotation axis O3in a direction shown by an arrow “a”, and presses the pulley56in a direction shown by an arrow D2.

The flange portion56b(i.e., the contact portion) of the pulley56has a tapered portion56a. When the intermediate transfer belt41is going to pass over the flange56b, the tapered portion56aguides the intermediate transfer belt41to its original position.

FIGS. 9A,9B and9C are perspective views showing an operation of the configuration at the end of the tension roller43.

FIG. 9Bshows a state in which the rotation axis O2of the tension roller43is parallel to the rotation axis O1of the drive roller42as shown inFIG. 7B. In this state, the intermediate transfer belt41moves stably.

FIG. 9Ashows a state in which the rotation axis O2of the tension roller43is inclined upward with respect to the rotation axis O1of the driving roller42as shown inFIG. 7A. In this state, the lever55rotates about the rotation axis O3(FIG. 8), and contacts the arm52.

FIG. 9Cshows a state in which the rotation axis O2of the tension roller43is inclined downward with respect to the rotation axis O1of the driving roller42as shown inFIG. 7C. In this state, the lever55rotates about the rotation axis O3(FIG. 8) to press the pulley56, so that the intermediate transfer belt41and the tension roller43are moved toward the bearing54R.

FIG. 10is a perspective view showing the configuration at the end of the tension roller43shown inFIGS. 9A through 9C.

The lever55has the rotation axis O3inclined at a predetermined angle with respect to the rotation axis O2of the tension roller43. The lever55has an elongated hole55aof substantially oval shape. The tension roller shaft43a(omitted inFIG. 10) penetrates the elongated hole55a, and is rotatably and slidably held in the elongated hole55a. The lever55has convex portions55bfacing the pulley56, and the convex portions55bare able to contact the pulley56. The above described bearing54L and the spring53L are provided in the rail portion52bof the arm52.

The lever55has the rotation axis O3inclined with respect to the rotation axis O2of the tension roller43. Therefore, when the left end (i.e., the pulley56side) of the tension roller43is shifted downward as shown inFIG. 7C, the lever55rotates downward and toward the tension roller43, and presses the pulley56.

When the left end (i.e., the pulley56side) of the tension roller43is shifted upward as shown inFIG. 7A, the lever55rotates upward and away from the tension roller43.

FIGS. 11A,11B,11C and11D are schematic views for illustrating the skew of the intermediate transfer belt41shown inFIG. 4.FIGS. 11A and 11Care plan views schematically showing a trajectory Xt of the intermediate transfer belt41together with the driving roller42and the tension roller43. InFIGS. 11A and 11C, left and right sides are reversed with respect toFIGS. 7A through 7C.FIGS. 11B and 11Dare side views schematically showing a trajectory Xt of the intermediate transfer belt together with the driving roller42and the tension roller43.

The intermediate transfer belt41is moved (rotated) by the driving roller42in the belt conveying direction X. If the driving roller42, the tension roller43an the backup roller43are not exactly parallel to one another, the intermediate transfer belt41may skew in a direction perpendicular to the belt conveying direction X when the intermediate transfer belt41moves.

For example, when the right end (i.e., the pulley56side) of the tension roller43shifts upward as shown inFIGS. 11A and 11B, the intermediate transfer belt41moves along the trajectory Xt shown inFIG. 11Adue to tendency of the intermediate transfer belt41to move perpendicularly to the axial direction of the tension roller43. As a result, the intermediate transfer belt41skews in a belt skew direction Y1perpendicular to the belt conveying direction X. By one rotation of the driving roller43, the intermediate transfer belt41skews in the belt skew direction Y1by an amount “m” shown inFIG. 11A. InFIG. 11A, a solid line indicates the trajectory Xt above the driving roller42and the tension roller43, and a dashed line indicates the trajectory Xt below the driving roller42and the tension roller43.

In contrast, when the right end (i.e., the pulley56side) of the tension roller43shifts downward as shown inFIGS. 11C and 11D, the intermediate transfer belt41skews in a belt skew direction Y2as shown inFIG. 11C.

The skew of the intermediate transfer belt41is caused by a non-parallelism of the driving roller42, the tension roller43and the backup roller44, an unevenness of the tension of the intermediate transfer belt41(for example, a difference in biasing force between springs53L and53R at both ends of the tension roller shaft43a), a difference in circumferential length between both lateral ends of the intermediate transfer belt41, a cylindrically of each of the rollers around which the intermediate transfer belt41is stretched (i.e., the driving roller42, the tension roller43and the backup roller44), and the like.

An entire operation of the image forming apparatus10will be described with reference toFIGS. 1 and 2.

InFIG. 1, the image forming control unit100of the image forming apparatus receives image data from the host device10A via the I/F control unit101, and starts an image forming operation. The image forming control unit100causes the feeding-conveying control unit109to drive the feeding motor115. The pickup roller12aof the medium feeding unit12is driven by the feeding motor115, and picks up the recording medium P from the medium tray11. The recording medium P picked up by the pickup roller12areaches a nip portion between the feeding roller12band the retard roller12c, and is separately fed.

The recording medium P fed by the medium feeding unit12then reaches the medium conveying unit13, and conveyed by the conveying roller pairs13a,13band13cto reach the transfer roller15as the secondary transfer portion.

The charging rollers32(32C,32M,32Y,32K) are applied with negative voltage (for example, −1000V) by the charge voltage control unit102, and charge the surfaces of the OPC drums31(31C,31M,31Y,31K) to negative potential (for example, −600V). The head control unit103causes the printing heads33(33C,33M,33Y,33K) to expose the surfaces of the OPC drums31(31C,31M,31Y,31K) according to the image data sent from the host device10A so as to form latent images on the OPC drums31.

The developing rollers34(34C,34M,34Y,34K) are applied with negative voltage (for example, −200V) by the developing voltage control unit104, and develop the latent images on the OPC drums31(31C,31M,31Y,31K) using the toners supplied by the toner supply units35(35C,35M,35Y,35K) so as to form toner images (i.e., visualized images) as developer images. The transfer rollers14(14C,14M,14Y,14K) as the primary transfer portions are applied with positive voltage (for example, +1500V) by the primary transfer voltage control unit105. The toner images formed on the OPC drums31(31C,31M,31Y,31K) are transferred to the intermediate transfer belt41at the nip portions between the OPC drums31and the transfer rollers14, so that the charged toner image is formed on the intermediate transfer belt41. In this regard, the backup roller44is connected to a frame ground (i.e., grounded).

The OPC drums31of the toner image forming units30(30C,30M,30Y,30K) and the intermediate transfer belt41are driven in synchronization with each other under control of the image forming control unit100, and toner images of the respective colors are transferred to the intermediate transfer belt41. The toner image formed on the intermediate transfer belt41is carried to the transfer roller15as the secondary transfer portion by the intermediate transfer belt41. The transfer roller15is applied with positive voltage (for example, +3000V) by the secondary transfer voltage control unit106. The toner image is transferred from the intermediate transfer belt41to the recording medium P by electric field formed by the transfer roller15and the grounded backup roller44.

The recording medium P (to which the toner image has been transferred by the transfer roller15) is conveyed to the fixing portion16. The fixing portion16applies heat and pressure to the recording medium P so as to melt and fix the toner image to the recording medium P. Then, the recording medium P is ejected by the ejection roller pairs17a,17band17cto the stacker portion18.

<Operation of Transfer Belt Unit>

An operation of the transfer belt unit40according to the first embodiment will be described with reference toFIGS. 7A through 10.

There is a case where the tension roller43is inclined as shown inFIG. 7C, due to flatness of an installation surface of the image forming apparatus10, a deflection of the frames51L and51R, an assembly error, a dimensional error or the like. In such a case, as shown inFIG. 8, the tension roller shaft43aof the tension roller43is also inclined, and therefore the lever55(with the elongated hole55athrough which the tension roller shaft43penetrates) contacts the tension roller shaft43at a position E1on a periphery of the elongated hole55a. The lever55is applied with a force in a direction shown by an arrow D1(i.e., downward) at the position E1. Therefore, the lever55rotates in a direction indicated by an arrow “a” about the rotation axis O3fixed to the frame51L.

The pulley56is provided between the lever55and the tension roller43so as to be movable along the tension roller43ain the axial direction. When the lever55rotates in the direction indicated by the arrow “a”, the lever55contacts the pulley56at the position E2. As the lever55contacts the pulley56, the lever55applies a force to the pulley56in a direction indicated by an arrow D2. Therefore, the pulley56slides along the tension roller shaft43asubstantially in the direction indicated by the arrow D2.

The intermediate transfer belt41contacts the flange portion56bof the pulley56at a position E3. When the pulley56moves along the tension roller shaft43a, the intermediate transfer belt41is applied with a force in a direction indicated by an arrow D3at the position E3. Therefore, the intermediate transfer belt41is moved toward the bearing54R side.

In this state, when the driving motor110starts rotating the driving roller42, the intermediate transfer belt41and the tension roller43rotate accompanying the rotation of the driving roller42. Accordingly, the intermediate transfer belt41skews in the belt skew direction Y2(seeFIG. 11C), and the intermediate transfer belt41presses the pulley56having the flange56bcontacting the lateral end of the intermediate transfer belt41at the positions E3and E4as shown inFIG. 8. The intermediate transfer belt41presses the pulley56with a force F in a direction opposite to the direction D3.

As a result, the pulley56slides along the tension roller shaft43ain the axial direction, i.e., the belt skew direction Y2. As the pulley56slides along the belt skew direction Y2, the lever55is pressed in a direction opposite to the direction D2, and the lever55rotates in a direction indicated by an arrow b. As the lever55rotates, the tension roller shaft43ais pressed by the elongated hole55aof the lever5to move in a direction (i.e., upward) opposite to the direction D1.

In this state, the arm52(FIG. 3) supporting the bearing54L rotates in a direction indicated by an arrow f about the rotation axis52a, and the bearing54L moves toward a position shown inFIG. 7B. Theoretically, the intermediate transfer belt41stably moves in the state shown inFIG. 7B. Practically, the intermediate transfer belt41stably moves in a state where weights of the intermediate transfer belt41and the arm52, friction forces between the respective parts and the like are balanced.

In the state shown inFIG. 7B, rotation axis O2of the tension roller43is substantially parallel to the rotation axis O1of the driving roller42. Therefore, if the rotation axis O1of the driving roller42is parallel to the rotation axis of the backup roller44, the skew of the intermediate transfer belt41decreases, and the intermediate transfer belt41stably moves in the state shown inFIG. 7B.

In contrast, when the tension roller43is inclined as shown inFIG. 7A, the intermediate transfer belt41skews in a direction indicated by a belt skew direction Y1, and the pulley56moves in the belt skew direction Y1. The lever55rotates downward about the rotation axis O3, and the convex portions55bpress the pulley56downward, so that the pulley56moves toward the position shown inFIG. 7B. The intermediate transfer belt41stably moves in this state.

The inclination operation of the tension roller43has been described with reference the operation fromFIG. 7CtoFIG. 7B(i.e., case 1), and the operation fromFIG. 7AtoFIG. 7B(i.e., case 2). Regardless of the direction in which the tension roller43is inclined, the lever55causes the tension roller43to be inclined so as to correct the skew of the intermediate transfer belt41.

For example, even if the intermediate transfer belt41and the tension roller43are not correctly mounted to predetermined positions in an assembling process of the transfer belt unit30, the intermediate transfer belt41is brought into a state where the intermediate transfer belt41stably moves without skew) due to thrust forces acting on the pulley56and the lateral end of the intermediate transfer belt41in the belt skew directions Y1and Y2, once the intermediate transfer belt41starts to move.

As described above, the rotation axis O2of the tension roller43and the rotation axis O1of the driving roller42and the rotation axis of the backup roller44become substantially parallel, and the skew of the intermediate transfer belt41is reduced, with the result that the intermediate transfer belt41stably moves without skew. In this state, the lateral end of the intermediate transfer belt41and the pulley56can be kept in contact with each other with a small contact force.

Next, a description will be made of a friction force (load) between the inner circumferential surface of the intermediate transfer belt41and the outer surface of the tension roller43during the inclination operation of the tension roller43with reference toFIGS. 12,13and14.

Hereinafter, the axial direction of the tension roller43(which is the same as the widthwise direction of the intermediate transfer belt41) will be also referred to as a widthwise direction.

FIG. 12is a schematic view showing a state of the inner circumferential surface of the intermediate transfer belt41and the outer surface of the tension roller43when the tension roller43is inclined. The tension roller43ofFIG. 12is not divided into a plurality of roller parts.

When the tension roller43is inclined about a inclination center (i.e., fulcrum) O1a, the tension roller is rotated by contact with the inner circumferential surface of the intermediate transfer belt41. Since the tension roller43has a length extending over a large portion of the width of the intermediate transfer belt41, a slippage occurs between the outer surface of the tension roller43and the inner circumferential surface of the intermediate transfer belt41.

In this state, there is a difference in slippage amount between a position closer to the inclination center O1aand a position farther from the inclination center O1a. When a widthwise center R2C of the tension roller43(i.e., a center in the axial direction of the tension roller43) rotates along a trajectory R2about the inclination center O1a, the outer surface of the tension roller43and the inner circumferential surface of the intermediate transfer belt41rotate relative to each other about the widthwise center R2C to form slippage portions60(on the assumption that no slippage occurs at the widthwise center R2C).

That is, a friction force is generated between the inner circumferential surface of the intermediate transfer belt41and the outer surface of the tension roller43. In such a case, the inclination operation of the tension roller43is not smoothly performed, and the skew correction (having been described with reference toFIGS. 7A through 9C) is not satisfactorily performed.

FIG. 13is a schematic view showing a state where a slippage occurs at the widthwise center R2C of the tension roller43in such a manner that the outer surface of the tension roller43rotates relative to the inner circumferential surface of the intermediate transfer belt41. The tension roller43ofFIG. 13is not divided into a plurality of roller parts.

InFIG. 13, a width of a roller body (i.e., except the tension roller shaft43a) of the tension roller43is expressed as B. The friction force between the outer surface of the driving roller43and the inner circumferential surface of the intermediate transfer belt41per unit length is expressed as S. Here, it is assumed that a stretching force and a friction force applied to the tension roller43in the width direction due to the tension of the intermediate transfer belt41are both constant. A moment generated at the widthwise center R2C of the tension roller43is expressed as Mc. A moment generated at a center O3a(i.e., right end center O3a) at the right end of the tension roller43is expressed as Ms. Here, it is assumed that the right end center O3ais an inclination center of the tension roller43. A friction force between the outer surface of the tension roller43and the inner circumferential surface of the intermediate transfer belt41is expressed as F.

The friction force generated equally at both left and right portions of the tension roller43is expressed as follows:
F=(B/2)×S(1)

This friction force F is generated at left and right portions each at a distance r=B/4 from the widthwise center R2C of the tension roller43, assuming that the friction force is evenly distributed in the widthwise direction. The moment Mc is expressed as follows:
Mc=2×F×r
Mc=(¼)×B2×S(2)

A distance r from the widthwise center R2C to the right end center O3ais set to 2/L (i.e., r=L/2). Using the distance r, the moment Ms about the right end center O3ais expressed as follows:
Ms=Mc/r
Ms=(2/B)×Mc
Ms=(½)×B×S(3)

Next, description will be made of the friction force between the inner circumferential surface of the intermediate transfer belt41and the outer surface of the tension roller43according to the first embodiment, i.e., the tension roller43which is evenly divided in five roller parts.

FIG. 14is a schematic view showing a state where friction forces are generated at widthwise centers R3-1, R3-2, R3-3, R3-4and R3-5of the respective roller parts43-1,43-2,43-3,43-4and43-5in such a manner that the outer surfaces of the roller parts43-1through43-5rotate relative to the inner circumferential surface of the intermediate transfer belt41.

The tension roller43divided into the roller parts43-1through43-5is inclined about the right end center O3aas was described with reference toFIGS. 12 and 13. Here, it is assumed that the outer surfaces of the roller parts43-1through43-5rotate without slippage on the inner circumferential surface of the intermediate transfer belt41at the widthwise centers of the roller parts43-1through43-5. In this case, slippages occur between the outer surfaces of the respective roller parts43-1through43-5and the inner circumferential surface of the intermediate transfer belt41in such a manner that the outer surfaces of the roller parts43-1through43-5rotate about the widthwise centers R3-1through R3-5.

InFIG. 14, a width of the roller body (i.e., the roller parts43-1through43-5) of the tension roller43is expressed as B. A division number (i.e., the number of roller parts) is expressed as t. A friction force between the outer surface of the driving roller43and the inner circumferential surface of the intermediate transfer belt41per unit length is expressed as S. Here, it is assumed that a stretching force and a friction force applied to the tension roller43in the width direction due to the tension of the intermediate transfer belt41are both constant. Moments generated at the widthwise centers R3-1, R3-2, R3-3, R3-4and R3-5of the roller parts43-1through43-5are expressed as Mc. A moment generated by the moments Mc at the right end center O3a(assumed to the inclination center of the tension roller43) is expressed as Ms. Friction forces between the outer surfaces of the roller parts43-1through43-5and the inner circumferential surface of the intermediate transfer belt41are expressed as F.

The friction force generated equally at both left and right portions of each of the roller parts43-1through43-5is expressed as follows:
F=B×S/(2×S)  (4)

This friction force F is generated at left and right portions each at a distance r from each of the widthwise centers R3-1through R3-5, assuming that the friction force is evenly distributed in the widthwise direction. The distance r, and the moment Mc generated at the distance r are expressed as follows:
r=B/4×t
Mc=2×F×r
Mc=B2×S/(4×t2)  (5)

The moment Ms about the right end center O3aof the tension roller43will be determined as follows. Here, N represents the division number (i.e., the number of roller parts of the tension roller43).

A distance rnfrom the right end center O3a(i.e., a center of the moment Ms) to each of the widthwise centers R3-1through R3-5(i.e., centers of the moments Mc) is expressed as follows:
rn=B{(k−1)/t+(1/(2×t))}

Then, the following equations are obtained:

Here, the above described equation (5) is substituted into the equation (f), and the following equation is obtained:

Therefore, the following equations are obtained:

When the division number t=1 is substituted into the equation (7), the following equation is obtained:
Ms=(½)×B×S

This is the same equation as the above described equation (3).

When the division number t=5 is substituted into the equation (7), the following equation is obtained:
Ms=½×B×S×⅕×(1+⅓+⅕+ 1/7+ 1/9)
Ms=½×B×S×563/1575

Therefore, when the division number t is 5, the moment Ms can be reduced by approximately 36% as compared with when the division number t is 1.

Table 1 shows the moments Ms for the division numbers 1 to 10 determined based on the equation (7), as compared to 100% for the moment Ms when the division number t is 1.

FIG. 15is a graph showing a relationship between the division number t of the tension roller43and the ratio of the moment Ms caused by the friction.

InFIG. 15, a horizontal axis indicates the division number t. A vertical axis indicates a ratio of the moment Ms (for the division numbers 1 to 10) with respect to the moment Ms (100%) for the division number 1.

According toFIG. 15, a point of inflection of a curve of the ratio of the moment Ms is located in the vicinity of a point where the division number t is 3.3. This means that the effect of the first embodiment is more effectively achieved when the division number t is greater than or equal to 4.

Theoretically, the effect of the first embodiment is achieved more effectively as the division number (t) increases. However, in practice, it is preferable that the width of the each of the roller parts43-1through43-5of the tension roller43is greater than or equal to 30 mm. This is because, if the width of the roller part is less than 30 mm, there is a possibility that a backlash may occur between the tension roller43and the tension roller shaft43aand may increase a load on the tension roller43.

The upper limit of the division number t is determined by a maximum sheet size of the recording medium P used in the image forming apparatus10. For example, if the maximum sheet size of the recording medium P used in the image forming apparatus10is A3 size, the width L of the tension roller43is determined to be approximately equal to the sheet width of 297 mm plus 40 mm. If the maximum sheet size of the recording medium P used in the image forming apparatus10is A4 size, the width L of the tension roller43is determined to be approximately equal to the sheet width of 210 mm plus 40 mm.

That is, when the image forming apparatus10is configured to use the recording medium P of up to A3 size, the division number t of the tension roller43is preferably less than or equal to 10. When the image forming apparatus10is configured to use the recording medium P of up to A4 size, the division number t of the tension roller43is preferably less than or equal to 8.

As a result, when the image forming apparatus10is configured to use the recording medium P of up to A3 size, the division number t of the tension roller43is preferably in a range from 4 to 10. When the image forming apparatus10is configured to use the recording medium P of up to A4 size, the division number t of the tension roller43is preferably in a range from 4 to 8.

As described above, as the tension roller43is divided in the axial direction into a plurality of roller parts43-1through43-5, it becomes possible to reduce the load on the tension roller43due to the friction between the outer surface of the tension roller43and the inner circumferential surface of the intermediate transfer belt41during the inclination operation.

To be more specific, since the friction force between the tension roller43and the intermediate transfer belt41is dispersed, the contact force between the flange portion56band the intermediate transfer belt becomes constant. Therefore, when the intermediate transfer belt41is guided to a stable position by the flange portion56bof the pulley56(in the case where the intermediate transfer belt41skews), it becomes possible to prevent the intermediate transfer belt41from being deformed by excessive load to pass over the flange56b.

The above description has been made on the assumption that the slippage between the tension roller43and the intermediate transfer belt41does not occur at the widthwise center R2C of the tension roller43. However, a portion where the slippage does not occur can be located on any other position on the rotation axis O2of the tension roller43.

According to the transfer belt unit40, the tension roller43is divided in the axial direction into a plurality of the roller parts43-1through43-5, and the roller parts43-1through43-5are independently rotatable. Therefore, it becomes possible to reduce the friction between the outer surface of the tension roller43and the inner circumferential surface of the intermediate transfer belt41during the inclination operation. Accordingly, the tension roller43can smoothly perform the inclination operation with small load. Thus, the contact force (stress) between the lateral end of the intermediate transfer belt41and the pulley56can be reduced. As a result, a lifetime of the transfer belt unit40can be lengthened.

Second Embodiment

FIGS. 16A and 16Bare schematic views showing the tension roller43according to the first embodiment and a tension roller43A (as a first rotation member) according to the second embodiment of the present invention both in assembled state.FIG. 17shows the tension roller43A of the second embodiment shown inFIG. 16B.

The transfer belt unit of the second embodiment is the same as the transfer belt unit40of the first embodiment except the tension roller43(43A).

As shown inFIG. 16A, the tension roller43(the roller parts43-1through43-5) of the first embodiment has a straight shape. That is, the outer diameter G1at the center of the tension roller43is the same as the outer diameter G1at the end of the tension roller43. In contrast, in the second embodiment, as shown inFIG. 16B, the outer diameter G3at the center of the tension roller43A (the roller parts43A-1through43A-5) is larger than the outer diameter G2at both ends of the tension roller43A.

More specifically, the tension roller43A of the second embodiment has a crown shape such that the outer diameter G3at the center of the tension roller43A is slightly larger than the outer diameter G2at both end of the tension roller43A.

A difference between the outer diameters G2and G3at both ends of the tension roller43is determined taking into consideration a deflection of the tension roller shaft43acaused when the tension is applied to the intermediate transfer belt41by the springs53L and53R.

As shown inFIG. 17, the tension roller shaft43apenetrates through the roller parts43A-1through43A-5of the tension roller43A to rotatably support the roller parts43A-1through43A-5. The roller parts43A-1through43A-5have ring-shaped boss portions43Ab-1,43Ab-2,43Ab-3,43Ab-4and43Ab-5, and form gaps43Ad between adjacent roller parts43A-1through43A-5. The outer diameter G2at both ends of the tension roller43A is smaller than the outer diameter G3at the center of the tension roller43A as described above. With the provision of the gaps43Ad, the roller parts43Ab-1through43Ab-5do not interfere with each other, even when the tension roller shaft43ais deflected by a force as shown by an arrow E. Further, when the deflection of the tension roller shaft43aoccurs, outer surfaces of the roller parts43Ab-1through43Ab-5on a side opposite to the driving roller42(shown by a line F inFIG. 17) are aligned substantially straightly as shown inFIG. 16B.

Operations of the image forming apparatus10and the transfer belt unit40of the second embodiment are the same as those of the first embodiment.

An operation of the tension roller43A of the second embodiment will be described. The tension roller43ofFIG. 16has a straight shape and is divided into a plurality of roller parts, as was described in the first embodiment. In this case, when tension roller shaft43ais deflected due to the tension of the intermediate transfer belt41applied by the springs53L and53R, there arises a difference between a stretching force T1(per unit width) at the end of the tension roller43and a stretching force T2(per unit width) at the center of the tension roller43.

Since the tension roller43(FIG. 16A) is divided into a plurality of roller parts, a bending strength of the tension roller43as a whole is relatively low. Therefore, the difference between the stretching forces T1and T2becomes relatively large. Depending on the strength of the tension roller shaft43aand the spring forces of the springs53L and53R, large stretching forces may be intensively generated at the ends of the tension roller43. In such a case, a tensile stress at the lateral end of the intermediate transfer belt41in the circumferential direction may increase, and the lifetime of the intermediate transfer belt41may be reduced.

As a countermeasure, it is possible to enhance a rigidity of the tension roller shaft43aby, for example, increasing the outer diameter of the tension roller shaft43aor using a hollow shaft. However, in such a case, a weight of the tension roller shaft43amay increase, or a manufacturing cost may increase.

In contrast, according to the second embodiment, the outer diameter G2at both ends of the tension roller43A (the roller parts43A-1through43A-5) is smaller than the outer diameter G3at the center of the tension roller43A as described above. Therefore, as shown inFIG. 16B, it becomes possible to reduce a difference between a stretching force T3(per unit width) at the end of the tension roller43A and a stretching force T4(per unit width) at the center of the tension roller43A.

According to the second embodiment, the tension roller43A is divided into a plurality of roller parts, and has a shape such that the outer diameter G3at the center is larger than the outer diameter G2at the end. Therefore, the stretching force T3(per unit width) at the end of the tension roller43A can be reduced, and a difference between the stretching force T3at the end of the tension roller43A and the stretching force T4at the center of the tension roller43A can be reduced. Accordingly, the intermediate transfer belt41becomes able to smoothly move. Further, since the tensile stress at the lateral ends of the intermediate transfer belt41can be reduced, the lifetime of the transfer belt unit40can be lengthened.

Following modifications can be made to the above described embodiments.

In the first and second embodiments, it is described that the belt driving device is used as the transfer belt unit40employed in the electrophotographic printer. However, the belt driving device of the present invention can be employed in other image forming apparatuses such as a copier, a facsimile machine or the like that form an image on the recording medium using electrophotography.

In the first and second embodiments, it is described that the belt driving device is employed in the image forming apparatus10of the intermediate transfer type that forms a developer image on the intermediate transfer belt41and transfers the developer image to the recording medium P. However, the belt driving device of the present invention can be applicable to a direct transfer type image forming apparatus that forms a developer image on the OPC drum31, and directly transfer the developer image from the OPC drum to the recording medium P.

In the first and second embodiments, it is described that the belt driving device is used as the transfer belt unit40employed in the electrophotographic image forming apparatus. However, the belt driving device of the present invention can also be employed in a fixing unit and a medium conveying device using an endless belt. Further, the belt driving device of the present invention can be used for other purposes than the electrophotographic image forming apparatus as long as an endless belt (i.e., a stretched member) is used.

In the first and second embodiment, the endless belt (more specifically, the intermediate transfer belt) has been described as an example of a stretched member. However, it is also possible to use other stretched members such as an ended (i.e., non-endless) belt, an endless sheet, an ended sheet or the like.

FIG. 18Ashows a tension roller43B according to a modification of the second embodiment. Although the tension roller43A of the second embodiment (seeFIGS. 16B and 17) has the crown shape, the tension roller43B of this modification (FIG. 18) has a tapered shape, and the outer diameter gradually increases from each end toward the center of the tension roller43B in such a manner that a difference between diameters at opposing ends of adjacent roller parts is minimized.

For comparison,FIG. 18Bschematically shows the crown shape of the tension roller43A of the second embodiment (FIGS. 16B and 17), andFIG. 18Cschematically shows the tapered shape of the tension roller43B of the modification (FIG. 18A). As shown inFIG. 18B, the tension roller43A of the second embodiment has the crown shape whose outer periphery has a continuous smooth curve C along the axial direction. As shown inFIG. 18C, the tension roller43B of the modification has a tapered shape whose outer periphery includes a plurality of straight tapers T. If the tension roller43B includes odd number of roller parts, the center roller part has a cylindrical shape. Using the tension roller43B having the tapered shape as shown inFIGS. 18A and 18C, the same advantages as in the second embodiment can be achieved.

Moreover, the features of the tension roller43in the first and second embodiments can also be applied to the backup roller44and/or the driving roller42.

For example,FIG. 19shows a modification in which the feature (FIGS. 16B and 17) of the second embodiment is applied to the driving roller42.

The driving roller42A shown inFIG. 19is divided into a plurality of roller parts. More specifically, the driving roller42A is divided into a roller part40cat the center of the driving roller42, and roller parts40don both sides of the roller part40c. The roller part40cis fixed to a driving roller shaft42b, and has a circumferential surface of high friction. The roller parts40dare rotatably supported by the driving roller shaft42b, and each roller part40dhas a tapered shape such that the outer diameter increases toward the roller part40c. With such a modification, the advantages described in the second embodiment can be achieved.

FIGS. 20A and 20Bare enlarged views showing modifications of configurations at the end portion of the tension roller43. As shown inFIG. 20A, a reinforcing member41acan be provided at the lateral end of the intermediate transfer belt41. Further, as shown inFIG. 20B, a guide member41bcan be provided on the inner circumferential surface at the lateral end of the intermediate transfer belt41. In this case, the pulley56is provided with a groove56cengaging the guide member41b. With such modifications, the advantages described in the first and second embodiments can be achieved.

While the preferred embodiments of the present invention have been illustrated in detail, it should be apparent that modifications and improvements may be made to the invention without departing from the spirit and scope of the invention as described in the following claims.