Patent ID: 12228871

DESCRIPTION OF THE EMBODIMENTS

First Embodiment

FIG.1is a sectional view of a printer (image forming apparatus)100using an electrophotography recording system and on which a fixing device1is mounted. A full-color toner image that is formed by superimposing toner images of four colors in an image forming section101is transferred by a transfer section102to a recording material P fed from a feeding unit. The toner image transferred to the recording material P is heat-fixed to the recording material at the fixing device1. The recording material P to which the toner image has been fixed is discharged to an output tray103. In duplex printing, after transferring and fixing the toner image to a first side of the recording material, the recording material is redirected and conveyed to a duplex conveying path104, so that an image is formed on a second side of the recording material by an operation that is similar to the operation that has been performed for forming the image on the first side. These image forming operations are known, so that they are not described in detail below.

FIG.2is a schematic sectional view of the fixing device1.FIG.3Ais a perspective view of an internal portion of the fixing device.FIG.3Bis a sectional view of the internal portion of the fixing device when the fixing device is seen from a recording-material discharging side. An arrow S represents a conveying direction of the recording material P, and a broken line X represents the center of the fixing device in a longitudinal direction. In the fixing device according to the embodiment, the broken line X is a conveyance reference of the recording material P. The recording material P is, regardless of its size, conveyed with its center in a width direction being aligned with the broken line X.

The fixing device1includes, for example, a heating unit2, a roller3that, along with the heating unit2, forms a fixing nip portion, and conveying rollers4that convey a recording material to which an image has been fixed. The heating unit2includes a flexible cylindrical rotary member (cylindrical belt, cylindrical film)9(hereunder referred to as “belt9”) and a heater5that heats the belt by contacting an inner surface of the belt9. The heating unit2further includes, for example, a heater holder6and a stay8. The heater holder6holds the heater5. The stay8is provided for maintaining the rigidity of the heating unit5. In the embodiment, the heater5, the heater holder6, and the stay8form a backup unit that contacts the inner surface of the belt9in a generatrix direction of the belt. A stretching roller is not provided at the inner surface of the belt9. Accordingly, the belt9is not stretched. The roller3has a rubber layer, and forms, along with the backup unit, a fixing nip portion N with the belt9disposed therebetween. The fixing nip portion N nips and conveys the recording material. The roller3is driven by a motor (not shown) via a gear61. The belt9is rotated by following the rotation of the roller3.

As shown inFIG.3A, U-shaped recesses for mounting two bearings of the roller3are provided in frames13of the fixing device. The two bearings that are provided at respective shaft end portions of the roller3are held in the recesses. Correcting mechanisms (may also be called “movement mechanisms”)10L and10R that correct the inclination of the belt9are provided at corresponding end portions of the backup unit. By disposing the correcting mechanisms at these positions, the correcting mechanisms10L and10R oppose the end surfaces of the belt9. The correcting mechanisms10L and10R are each provided with a holding member12(described later). By providing grooves12fof the holding members12at the U-shaped recesses of the frames13(seeFIG.4A), the heating unit2is held by the frames13similarly to the roller3. Compression springs7(first urging members) apply pressure to top surfaces12cof the holding members12(seeFIG.4A). The pressure applied by the springs7urges the heater5towards the roller3via the holding members12, the stay8, and the heater holder6. This causes the rubber layer of the roller3to be compressed and the backup unit and the roller3to form the fixing nip portion N with the belt9disposed therebetween. A recording material P that bears a toner image is nipped and conveyed to the fixing nip portion N while contacting the belt9. During this period, the toner image is heated by the heater5via the belt9, and is fixed to the recording material P.

The belt9according to the embodiment includes a base layer formed of heat-resistant resin (to be more specific, polyimide), a surface layer formed of fluorocarbon resin, and a rubber layer (silicone rubber layer) formed between the base layer and the surface layer. The material of the base layer may be a metal, such as stainless steel or nickel. The rubber layer may be left out if not required.

As shown inFIG.3B, the heater5may be elongated in a longitudinal direction of the fixing device (that is, the generatrix direction of the belt9). The heater5is a ceramic heater in which heating generating resistors are printed on a ceramic substrate. Electric power is supplied to the heater5via a connector62for supplying electric power. The temperature of the heater5is monitored by a temperature detecting element (not shown). The electric power supplied to the heater5is controlled so that the temperature detected by the temperature detecting element is maintained at a target temperature. The heater holder6is formed by molding heat-resistant resin, such as liquid crystal polymer (LCP) or polyphenylene sulfide (PPS). The heater holder6is provided with a groove for fitting the heater5thereto. By fitting the heater5to the groove, the heater5is held in the longitudinal direction. The stay8is U-shaped in cross section, and is formed of a metal (iron in the embodiment). The stay8contacts the holder6in the longitudinal direction, and reinforces the holder6.

Next, the correcting mechanism10R and the correcting mechanism10L that correct lateral shift of the belt9are described with reference toFIGS.4A to8B. The shape of the correcting mechanism10R and the shape of the correcting mechanism10L are substantially axially symmetrical with reference to a conveyance reference X of a recording material P. Therefore, only the correcting mechanism10R is described, and the correcting mechanism10L is not described.

FIG.4Ais a perspective view of the correcting mechanism10R.FIG.4Bis a sectional view of the correcting mechanism10R when seen from an upstream side in the conveying direction of a recording material.FIG.5Ais a perspective view of a movable member11(described later).FIG.5Bis a perspective view of a holding member12that holds the movable member11.FIG.6illustrates the correcting mechanism10R when seen from the direction of arrow VI inFIG.4B.FIGS.7A and7BandFIGS.8A and8Beach illustrate a mechanism for correcting the orientation of the belt by the correcting mechanisms.

The correcting mechanism10R includes the movable member11, the holding member12that holds the movable member11, and compression springs (second urging members)14that urge the movable member11. As mentioned above, the holding member12is fitted to the U-shaped recess of the frame13of the fixing device. This substantially determines the position of the holding member12in the longitudinal direction of the heater and the position of the holding member12in the recording material conveying direction. Since the holding member12is urged towards the roller3by the springs7, the holding member12is in a substantially secured state.

The movable member11is a part that is movably engaged with the holding member12. The movable member11is in contact with a cutaway portion provided at an end portion of the stay8in the longitudinal direction. A slight gap is provided between the holding member12and a top portion of the movable member11. As shown inFIG.4A, the movable member11has an end-surface opposing portion11athat opposes an end surface of the belt9. When the belt9is laterally shifted in the generatrix direction thereof, the end surface of the belt9collides with the end-surface opposing portion11a. The movable member11has an inner-surface opposing portion11cthat opposes an inner surface of an end portion of the belt9. A slight clearance is provided between the inner surface of the belt9and the inner-surface opposing portion11c. The inner-surface opposing portion11chas the function of guiding the inner surface of the belt9when the belt9rotates.

As shown inFIG.5A, the movable member11has a protrusion11bextending obliquely with respect to the longitudinal direction of the heater. As shown inFIG.5B, the holding member12has a recess (guide)12bextending obliquely with respect to the longitudinal direction of the heater. When the movable member11and the holding member12are combined, the protrusion11bof the movable member11is fitted in the recess12bof the holding member12. By virtue of this structure, the movable member11is slidably held along the recess12bof the holding member12. Reference numerals14denote the compression springs that urge the movable member11away from a seating surface12aof the holding member12.

Next, the operations of the correcting mechanisms10are described with reference toFIGS.6to8B.FIGS.6and7A each illustrate a state of the correcting mechanism in which the end surface of the belt9is not in contact with the end-surface opposing portion11a. When the belt9is rotated by following the rotation of the roller3, the belt9contacts the inner-surface opposing portion11cof the movable member11in an area that is disposed upstream of the heater5in a rotation direction of the belt. In contrast, in an area that is disposed downstream of the heater5in the rotation direction of the belt, the belt9is separated from the inner-surface opposing portion11cof the movable member11.

When the end surface of the belt9is not in contact with the end-surface opposing portion11a, the movable member11that is urged by the springs14is positioned at a location that is farthest from the seating surface12ain the holding member12. At this time, even if the protrusion11bof the movable member11collides with a first stopper12dof the holding member12and is urged by the springs14, the movable member11is positioned by restricting the movement of the movable member11.

As shown inFIG.7A, when the end surface of the belt9is not in contact with the end-surface opposing portion11a, the distance between the end surface of the belt9and the end-surface opposing portion11aof the movable member11is D1. The distance from the seating surface12aof the holding member12to the end-surface opposing portion11aof the movable member11is D2.

FIG.7Billustrates a state in which the end surface of the belt9contacts the end-surface opposing portion11aas a result of lateral shift of the belt9in the direction of arrow M1 and the belt9pushes the movable member11in the direction of arrow M1 against the urging force of the springs14.

When, for example, the belt9is laterally shifted towards the movable member11as a result of, for example, the roller3and the belt9being out of alignment with each other, the end surface of the belt9comes into contact with the movable member11. When the belt9is laterally shifted further, the belt9pushes the movable member in the direction of arrow M1 against the urging force of the springs14, so that the movable member11moves. Since the protrusion11bof the movable member11moves along the recess12bof the holding member12, the movable member11moves in the direction of arrow M2. When the protrusion11bcollides with a second stopper12gof the recess12b, the movable member11stops moving. As this time, as shown inFIG.7B, the distance from the seating surface12aof the holding member12to the end-surface opposing portion11aof the movable member11is D3 (<D2). Compared to the state inFIG.7A, the movable member11is moved through a distance D4 towards an upstream side in the recording material conveying direction S.

As mentioned above, when the belt9is rotating, the inner surface of the belt9is in contact with the inner-surface opposing portion11cof the movable member11. Therefore, when, as shown inFIG.7B, the movable member11is moved towards the upstream side in the recording material conveying direction S, the inner-surface opposing portion11cpushes the inner surface of the belt9, so that the end portion of the belt at the side of the correcting mechanism10R moves towards the upstream side in the recording material conveying direction S. In contrast, since the correcting mechanism10L that is positioned opposite to the correcting mechanism10R in the longitudinal direction of the heater is not pushed by an end surface of the belt9, the movable member of the correcting mechanism10L does not move.

When the movement direction of lateral shift of the belt9is in the opposite direction, that is, when the belt collides with the correcting mechanism10L, only the movable member in the correcting mechanism10L moves towards the upstream side in the recording material conveying direction S. This movement causes the end portion of the belt at the side of the correcting mechanism10L to move towards the upstream side in the recording material conveying direction S.

In this way, when the belt9is laterally shifted in the longitudinal direction of the heater (that is, the generatrix direction of the belt), and collides with one of the correcting mechanisms10R and10L, only the end portion of the belt9on the downstream side in a lateral shift direction receives a force towards the upstream side in the recording material conveying direction. Due to this principle, the state of alignment of the belt9with respect to the roller3is changed, the orientation of the belt is corrected, and the belt moves away from the movable member (that is, in a direction opposite to the direction of arrow M1 shown inFIG.7B), so that the force that is applied to the end surface of the belt9is restricted. This makes it possible to restrict breakage of the belt. As mentioned above, the movable member11is urged by the springs14. Therefore, when the belt9moves in a direction opposite to the direction of arrow M1 from the state shown inFIG.7B, the movable member11is pushed back to the position shown inFIG.7Aor to a position between the positions shown inFIGS.7A and7B.

Next, the principle of reducing stress that is applied to the end surfaces of the belt9is further described with reference toFIGS.8A and8B.FIGS.8A and8Beach illustrate the heating unit2and the roller3when seen from the side of the belt9.FIG.8Aillustrates a state in which the belt is laterally shifted.FIG.8Billustrates a state in which the belt is no longer laterally shifted.

In general, lateral shift of the belt9in the generatrix direction is caused by the roller3and the belt9being out of alignment with each other.FIG.8Aillustrates a state in which the roller3and the belt9are out of alignment with each other. That is,FIG.8Aillustrates a state in which the end portion of the belt at the side of the correcting mechanism10R is inclined towards the downstream side in the recording material conveying direction S and in which the end portion of the belt at the side of the correcting mechanism10L is inclined towards the upstream side in the recording material conveying direction S. As shown inFIG.8A, a force F is applied to the belt9due to the rotation of the roller3. The force F can be broken down into a force F1 in the generatrix direction of the belt9and a force F2 in a direction that is orthogonal to the generatrix direction. The belt9is laterally shifted towards the correcting mechanism10R by the force F1. When the belt9contacts and pushes the movable member11of the correcting mechanism10R, the movable member11is guided to the holding member12and moves towards the upstream side in the recording material conveying direction S. The movement of the movable member11corrects the orientation of the belt9as shown inFIG.8Bon the basis of the aforementioned principle. Since the roller3and the belt9are no longer out of alignment, the angle between the force F and the generatrix direction of the belt9is changed. As a result, the force F1 is reduced, so that stress that is applied to the end surface of the belt9is also reduced.

The magnitude of the force F1 changes in accordance with the movement amount of the movable member11.FIG.9illustrates the relationship between a force for pushing the movable member11by the belt9and a force for pushing the movable member11by the springs14in accordance with the movement amount of the movable member11in the longitudinal direction of the heater. As shown inFIG.9, when the belt9starts pushing the movable member11, one of the end portions of the belt is pushed by the inner-surface opposing portion11cof the movable member, so that they gradually become aligned. That is, since the movement amount of the movable member towards the upstream side in the recording material conveying direction is increased as the movement amount of the movable member is increased, the amount of correction of the orientation (inclination) of the belt is increased, so that the force F1 is reduced. When the movement amount of the movable member is increased, the force for pushing the movable member11by the springs14is gradually increased. If the force when the belt9starts pushing the movable member11is small, that is, if the force F1 is small, the movable member11stops at a position where the force F1 and the force of the springs14are in equilibrium before a maximum movement amount (D2−D3) is reached (state 1). If the force when the belt9starts pushing the movable member11is large, that is, when the force F1 is large, the maximum movement amount (D2−D3) is reached before the force F1 and the force of the springs14are in equilibrium, and the movable member11stops at the position where the maximum movement amount is reached (state 2). A clearance is provided between the inner surface of the belt and the inner-surface opposing portion11cso that the state of contact between the inner-surface opposing portion11cand the inner surface of the belt is maintained even in the state in which the movable member11has moved by the maximum movement amount (D2−D3). That is, a clearance is provided between the inner surface of the belt and the inner-surface opposing portion11cso that the state of contact between the inner-surface opposing portion11cand the inner surface of the belt is maintained even in the state in which the movable member11has moved upstream in the recording material conveying direction through a distance D4.

As mentioned above, since it is possible to reduce stress that is applied to the end surfaces of the belt9, it is possible to suppress wear of the end surfaces of the belt9.

Although, in the embodiment, correcting mechanisms are provided at both opposing ends of the belt, the aforementioned correcting mechanism may be provided only at a side towards which the belt is laterally shifted, with the direction in which the belt is laterally shifted being previously set in one direction. In addition, in the embodiment, the length of the belt is assumed as being less than the span between the two movable members. However, the length of the belt may be about the same as the space between the two movable members, that is, the two ends of the belt may be constantly in contact with the two movable members. Further, although a structure in which the inner-surface opposing portion and the end-surface opposing portion are formed as one part serving as a movable member is described, the inner-surface opposing portion and the end-surface opposing portion may be separate parts. This applies to the other embodiments described below.

Second Embodiment

Next, a fixing device according to a second embodiment is described while focusing on the differences from the first embodiment.FIG.10Ais a perspective view of a movable member21.FIG.10Bis a perspective view of a holding member22that holds the movable member21. Further,FIG.11illustrates a correcting mechanism20R, which is one of the two correcting mechanisms, when seen from a direction that is the same as the direction of arrow VI shown inFIG.4B.FIGS.12A and12Beach illustrate a mechanism for correcting the orientation of a belt9by the correcting mechanism20R.

The correcting mechanism20R includes a movable member21, a holding member22that holds the movable member21, an extension spring24that urges the movable member21, and a link member25.

The movable member21includes an end-surface opposing portion21a, protrusions21b, and an inner-surface opposing portion21c. The end-surface opposing portion21acollides with an end surface of the belt when the belt9is laterally shifted. The inner-surface opposing portion21copposes an inner surface of the belt in a generatrix direction thereof. Further, the movable member21includes a protrusion21dand a supporting portion21eof the extension spring24. The protrusion21drotatably holds the link member25(described later).

The holding member22that holds the movable member21has a surface22aand recesses22b. The surface22ais substantially parallel to the end-surface opposing portion21aof the movable member21. The recesses22bguide the protrusions21bof the movable member21. The holding member22further has a protrusion22d, a supporting portion22eof the extension spring24, and grooves22f. The protrusion22dserves as a rotational center of the link member25. The grooves22fare provided for fitting the holding member22to a U-shaped recess of a device frame13. The link member25is mounted so as to link the protrusion21dand the protrusion22d.

Next, the operation of the correcting mechanism20R is described. As shown inFIG.12A, when an end surface of the belt9is not in contact with the end-surface opposing portion21a, the distance between the end surface of the belt9and the end-surface opposing portion21aof the movable member21is D1. The distance from the surface22aof the holding member22to the end-surface opposing portion21aof the movable member21is D2.

FIG.12Billustrates a state in which the end surface of the belt9contacts the end-surface opposing portion21aas a result of lateral shift of the belt9in the direction of arrow M1 and the belt9pushes the movable member21in the direction of arrow M1 against the urging force of the spring24. When the belt9pushes the movable member21, the protrusions21bmove in the direction of arrow M3 while being guided by the recesses22b. During this movement, the link member25rotates around the protrusion22d. By the action of the link member25, the movable member21moves parallel to the direction of arrow M3 without changing its orientation from the state shown inFIG.12A. Then, when the protrusions21bhave moved to end portions of the recesses22b, the movable member21stops moving. At this time, as shown inFIG.12B, the distance from the surface22aof the holding member22to the end-surface opposing portion21aof the movable member21is D3 (<D2). Compared to the state shown inFIG.12A, the movable member21is moved through a distance D4 towards an upstream side in a recording material conveying direction S.

When the movable member21moves towards the upstream side in the recording material conveying direction S, the inner-surface opposing portion21cpushes the inner surface of the belt9, as a result of which the end portion of the belt at the side of the correcting mechanism20R moves towards the upstream side in the recording material conveying direction S. In contrast, since a correcting mechanism20L (not shown) that is positioned opposite to the correcting mechanism20R in the longitudinal direction of a heater is not pushed by an end surface of the belt9, the movable member of the correcting mechanism20L does not move.

As described above, when the movable member moves, the alignment of the belt9changes with respect to the roller3on the basis of a principle that is the same as that used in the first embodiment, and the orientation of the belt is corrected. This causes the belt to move away from the movable member (that is, in a direction opposite to the direction of arrow M1 shown inFIG.12B), so that the force that is applied to the end surface of the belt9is restricted. This makes it possible to restrict breakage of the belt.

Third Embodiment

Next, a fixing device according to a third embodiment is described while focusing on the differences from the first and second embodiments.FIG.13is a perspective view of the fixing device.FIG.14Ais a perspective view of a movable member31.FIG.14Bis a perspective view of a holding member32that holds the movable member31. Further,FIG.15Ais a perspective view of an end portion of a link member36(described later).FIG.15Billustrates a correcting mechanism30R, which is one of the two correcting mechanisms, when seen from a direction that is the same as the direction of arrow VI shown inFIG.4B.FIGS.16A and16Beach illustrate a mechanism for correcting the orientation of a belt9by the correcting mechanisms30R and30L.

The correcting mechanisms30R and30L each include a movable member31and a holding member32that holds the movable member31. A link member36that links the two movable members31is provided at the correcting mechanisms30R and30L.

Each movable member31includes an end-surface opposing portion31a, protrusions31b, and an inner-surface opposing portion31c. Each end-surface opposing portion31acollides with an end surface of the belt when the belt9is laterally shifted. Each inner-surface opposing portion31copposes an inner surface of the belt in a generatrix direction thereof. Further, each movable member31has a hole31dfor rotatably holding the link member36(described later).

Each holding member32that holds the corresponding movable member31has a surface32aand recesses32b. Each surface32ais substantially parallel to the end-surface opposing portion31aof the corresponding movable member31. Each recess32bguides the corresponding protrusion31bof the movable member31. Each holding member32further has a groove32ffor fitting the corresponding holding member32to a U-shaped recess of a device frame13.

The device according to the third embodiment includes the link member36that links the movable member of the correcting mechanism30R and the movable member of the correcting mechanism30L. The link member36includes a shaft36R that is inserted into the hole31dof the movable member of the correcting mechanism30R and a shaft36L that is inserted into the hole31dof the movable member of the correcting mechanism30L.

Next, the operation of the correcting mechanism30R and the correcting mechanism30L is described. As shown inFIG.16A, when end surfaces of the belt9are not in contact with the end-surface opposing portions31a, the distance between each end surface of the belt9and the end-surface opposing portion31aof its corresponding movable member31is D1. The distance from the surface32aof each holding member32to the end-surface opposing portion31aof its corresponding movable member31is D2.

FIG.16Billustrates a state in which an end surface of the belt9contacts the end-surface opposing portion31aof the movable member of the correcting mechanism30R as a result of lateral shift of the belt9in the direction of arrow M1 and the belt9pushes the movable member31in the direction of arrow M1. When the belt9pushes the movable member31, the movable member of the correcting mechanism30R moves in the direction of arrow M4 while the protrusions31bare guided by the recesses32b. The movable member of the correcting mechanism30L and the movable member of the correcting mechanism30R are linked by the link member36. The two movable members move with each other's movement. Therefore, when the movable member of the correcting mechanism30R moves in the direction of arrow M4, the movable member of the correcting mechanism30L moves in the direction of arrow M5. That is, when the movable member of the correcting mechanism30R moves upstream in a recording material conveying direction, the movable member of the correcting mechanism30L moves downstream in the recording material conveying direction.

InFIG.16B, a distance D3 is a distance from the surface32ato the end-surface opposing portion31awhen the protrusions31bhave moved to end portions of the recesses32b. At this time, the movement distances of the two movable members in the recording material conveying direction are both D4. When the belt9is laterally shifted towards the correcting mechanism30L, the movement directions of the two movable members in the recording material conveying direction are opposite to the directions shown inFIG.16B.

By virtue of the above-described structure, compared to the structure in which only one of the movable members is moved, the inclination of the belt9in the direction of correction of the lateral shift of the belt is increased, so that the ability to correct the lateral shift of the belt is increased.

Fourth Embodiment

Next, a fixing device according to a fourth embodiment is described while focusing on the differences from the first embodiment to the third embodiment.

A correcting mechanism according to the fourth embodiment includes a sensor46that detects lateral shift of a belt9, and moves a movable member upstream in a recording material conveying direction by power of a motor (driving section) that is in accordance with an output of the sensor46.

FIG.17Ais a perspective view of a correcting mechanism40L.FIG.17Billustrates the correcting mechanism40L when seen from above the correcting mechanism40L. A correcting mechanism40R that is disposed at the opposite side also has the same structure.FIGS.18A and18Billustrate the operation of the correcting mechanism.

The photosensor46is disposed above the movable member41. The sensor46detects the movement of the movable member41in a generatrix direction of the belt. When the belt9is not in contact with the movable member41and the movable member is not moving, the movable member41is at a position shown inFIG.18A, and reflection light from a light source provided at the sensor46is not reflected by the sensor. However, when the movable member41moves in the generatrix direction of the belt by the lateral shift of the belt9, the movable member41moves to the position shown inFIG.18B, and the reflection light from the light source is detected by the sensor46. In accordance with this output, a motor (not shown) rotates a gear40RG that engages with a gear41hG provided at a rack41hof the movable member41, to move the movable member41in the direction of arrow M6, that is, upstream in the recording material conveying direction.

This causes the alignment of the belt9with respect to the roller3to change on the basis of a principle that is the same as that used in the first embodiment, and the orientation of the belt is corrected. This causes the belt to move away from the movable member, so that the force that is applied to the end surface of the belt9is restricted.

In the fourth embodiment, the movable member may be moved in the direction of arrow M6 before the end surface of the belt comes into contact with the end-surface opposing portion of the movable member.

Fifth Embodiment

Next, a fixing device according to a fifth embodiment is described while focusing on the differences from the first embodiment to the fourth embodiment.

A movable member according to the fifth embodiment differs from those of the other embodiments in that a portion thereof that pushes a belt upstream in a recording material conveying direction for correcting the orientation of the belt opposes an outer surface of the belt.FIG.19is a perspective view of a correcting mechanism50R of the device according to the fifth embodiment. The correcting mechanism50R includes a movable member51and a holding member52. The movable member51includes an outer-surface opposing portion51jthat opposes the outer surface of an end portion of the belt. When the belt is laterally shifted and pushes the movable member, the outer-surface opposing portion51jof the movable member urges the end portion of the belt towards an upstream side in the recording material conveying direction using a force resulting from the pushing. This causes the alignment of the belt9with respect to the roller3to change on the basis of a principle that is the same as that used in the first embodiment, and the orientation of the belt is corrected. This causes the belt to move away from the movable member, so that the force that is applied to the end surface of the belt9is restricted.

Sixth Embodiment

Next, correcting mechanisms110R and110L that correct the inclination of a belt9according to a sixth embodiment are described with reference toFIGS.20A to24D. The shape of the correcting mechanism110R and the shape of the correcting mechanism110L are substantially axially symmetrical with reference to a conveyance reference X of a recording material P. Therefore, the correcting mechanisms110R and110L are described by primarily describing the correcting mechanism110R and partly describing the correcting mechanism110L.

FIG.20Ais a perspective view of the correcting mechanism110L.FIG.20Bis a sectional view of the correcting mechanism110R when seen from an upstream side in a recording material conveying direction.FIG.21Ais a perspective view of a movable member111(described below).FIG.21Bis a perspective view of a holding member112that holds the movable member111. Further,FIG.22illustrates the correcting mechanism110R when seen from the direction of arrow XXII inFIG.20B.FIGS.23A and23BandFIGS.24A to24Deach illustrate a mechanism that corrects the orientation of the belt by the correcting mechanisms.

The correcting mechanism110R includes a movable member111, a holding member112that holds the movable member111, and compression springs (urging members)14that urge the movable member111. As described above, the holding member112is fitted to a U-shaped recess of a frame13of a fixing device. This causes the position of the holding member112in a longitudinal direction of a heater and the position of the holding member112in the recording material conveying direction to be substantially determined. Since the holding member112is urged towards a roller3by a spring7, the holding member112is in a substantially secured state.

The movable member111is a part that is movably engaged with the holding member112. The movable member111is in contact with a cutaway portion provided at an end portion of a stay8in a longitudinal direction. A slight gap is provided between the holding member112and a top portion of the movable member111. As shown inFIG.20A, the movable member111has an end-surface opposing portion111athat opposes an end surface of the belt9. When the belt9is laterally shifted in a generatrix direction thereof, the end surface of the belt9collides with the end-surface opposing portion111a. The movable member111has an inner-surface opposing portion111cthat opposes an inner surface of the end portion of the belt9. A slight clearance is provided between the inner surface of the belt9and the inner-surface opposing portion111c. The inner-surface opposing portion111chas the function of guiding the inner surface of the belt9when the belt rotates.

As shown inFIG.21A, the movable member111has a protrusion111bextending obliquely with respect to the longitudinal direction of the heater. As shown inFIG.21B, the holding member112has a recess (guide)112bextending obliquely with respect to the longitudinal direction of the heater. When the movable member111and the holding member112are combined, the protrusion111bof the movable member111is fitted in the recess112bof the holding member112. By virtue of this structure, the movable member111is slidably held along the recess112bof the holding member112. Reference numerals14denote the compression springs that urge the movable member111away from a seating surface112aof the holding member112.

Next, the operations of the correcting mechanisms110are described with reference toFIGS.22to24D.FIGS.22and23Aeach illustrate a state of the correcting mechanism in which the end surface of the belt9is not in contact with the end-surface opposing portion111a. When the belt9is rotated by following the rotation of the roller3, the belt9contacts the inner-surface opposing portion111cof the movable member111in an area that is disposed upstream of the heater5in a rotation direction of the belt. In contrast, in an area that is disposed downstream of the heater5in the rotation direction of the belt, the belt9is separated from the inner-surface opposing portion111cof the movable member111.

When the end surface of the belt9is not in contact with the end-surface opposing portion111a, the movable member111that is urged by the springs14is positioned at a location that is farthest from the seating surface112ain the holding member112. At this time, even if the protrusion111bof the movable member111collides with a first stopper112dof the holding member112and is urged by the springs14, the movable member111is positioned by restricting the movement of the movable member111.

As shown inFIG.23A, when the end surface of the belt9is not in contact with the end-surface opposing portion111a, the distance between the end surface of the belt9and the end-surface opposing portion111aof the movable member111is D1. The distance from the seating surface112aof the holding member112to the end-surface opposing portion111aof the movable member111is D2.

FIG.23Billustrates a state in which the end surface of the belt9contacts the end-surface opposing portion111aas a result of lateral shift of the belt9in the direction of arrow M1 and the belt9pushes the movable member111in the direction of arrow M1 against the urging force of the springs14.

When, for example, the belt9is laterally shifted towards the movable member111as a result of, for example, the roller3and the belt9being out of alignment with each other, the end surface of the belt9comes into contact with the movable member111. When the belt9is laterally shifted further, the belt9pushes the movable member in the direction of arrow M1 against the urging force of the springs14, so that the movable member11moves by making use of a force of lateral shift of the belt. Since the protrusion111bof the movable member111moves along the recess12bof the holding member112, the movable member111moves in the direction of arrow M2. When the protrusion111bcollides with a second stopper112gof the recess112b, the movable member111stops moving. As this time, as shown inFIG.23B, the distance from the seating surface112aof the holding member112to the end-surface opposing portion111aof the movable member111is D3 (<D2). Compared to the state inFIG.23A, the movable member111is moved through a distance D4 towards an upstream side in the recording material conveying direction S.

As mentioned above, when the belt9is rotating, the inner surface of the belt9is in contact with the inner-surface opposing portion111cof the movable member111. Therefore, when, as shown inFIG.23B, the movable member111is moved towards the upstream side in the recording material conveying direction S, the inner-surface opposing portion111cpushes the inner surface of the belt9, so that the end portion of the belt at the side of the correcting mechanism110R moves towards the upstream side in the recording material conveying direction S. In contrast, since the correcting mechanism110L that is positioned opposite to the correcting mechanism110R in the longitudinal direction of the heater is not pushed by an end surface of the belt9, the movable member of the correcting mechanism110L does not move.

When the movement direction of lateral shift of the belt9is in the opposite direction, that is, when the belt collides with the correcting mechanism110L, only the movable member in the correcting mechanism110L moves towards the upstream side in the recording material conveying direction S. This movement causes the end portion of the belt at the side of the correcting mechanism110L to move towards the upstream side in the recording material conveying direction S.

In this way, when the belt9is laterally shifted in the longitudinal direction of the heater (that is, the generatrix direction of the belt), and collides with one of the correcting mechanisms110R and110L, only the end portion of the belt9on the downstream side in a lateral shift direction receives a force towards the upstream side in the recording material conveying direction. Due to this principle, the alignment of the belt9with respect to the roller3is changed, the orientation of the belt is corrected, and the belt moves away from the movable member (that is, in a direction opposite to the direction of arrow M1 shown inFIG.23B), so that the force that is applied to the end surface of the belt9is restricted. This makes it possible to restrict breakage of the belt. As mentioned above, the movable member111is urged by the springs14. Therefore, when the belt9moves in a direction opposite to the direction of arrow M1 from the state shown inFIG.23B, the movable member111is pushed back to the position shown inFIG.23Aor to a position between the positions shown inFIGS.23A and23B.

Next, the principle of reducing stress that is applied to the end surfaces of the belt9is further described with reference toFIGS.24A to24C.FIGS.24A to24Ceach illustrate a heating unit2and the roller3when seen from the side of the belt9.FIG.24Aillustrates a state in which the belt is laterally shifted.FIG.24Billustrates a state in which the belt is no longer laterally shifted.FIG.24Cillustrates a state in which the inclination of the belt9has been corrected.

In general, lateral shift of the belt9in the generatrix direction is caused by the roller3and the belt9being out of alignment with each other.FIG.24Aillustrates a state in which the roller3and the belt9are out of alignment with each other. That is,FIG.24Aillustrates a state in which the end portion of the belt at the side of the correcting mechanism110L is inclined towards the downstream side in the recording material conveying direction S and in which the end portion of the belt at the side of the correcting mechanism110R is inclined towards the upstream side in the recording material conveying direction S. As shown inFIG.24A, a force F is applied to the belt9due to the rotation of the roller3. The force F can be broken down into a force F1 in the generatrix direction of the belt9and a force F2 in a direction that is orthogonal to the generatrix direction. The belt9is laterally shifted towards the correcting mechanism110L by the force F1. When the belt9contacts and pushes the movable member111of the correcting mechanism110L (FIG.24B), the movable member111is guided to the holding member112and moves towards the upstream side in the recording material conveying direction S. The movement of the movable member111corrects the orientation of the belt9as shown inFIG.24Con the basis of the aforementioned principle. Since the roller3and the belt9are no longer out of alignment, the angle between the force F and the generatrix direction of the belt9is changed. As a result, the force F1 is reduced (F1 to F1′), so that stress that is applied to the end surface of the belt9is also reduced.

As mentioned above, since it is possible to reduce stress that is applied to the end surfaces of the belt9, it is possible to suppress wear of the end surfaces of the belt9.

When the position of the center of a roller section of the pressure roller3in the longitudinal direction and the position of the center of a sheet S in a width direction are displaced from each other, conveying forces that are applied to the belt9as a result of rotation of the pressure roller3become nonuniform at both end portions of the belt9. For example, when, as shown inFIG.24D, the sheet S is displaced towards the side of the correcting mechanism110R, an area where the pressure roller3directly contacts the belt9is longer at the side of the correcting mechanism110L than at the side of the correcting mechanism110R. Friction force between the pressure roller3and the belt9is greater than friction force between paper and the belt9. Therefore, rotary force of the belt9generated by the pressure roller3is such that a rotary force Ff at the correcting mechanism110L is greater than a rotary force Fr at the correcting mechanism110R. As a result, the rotation of the end portion of the belt at the side of the correcting mechanism110R is delayed. Therefore, the end portion of the belt at the side of the correcting mechanism110R moves towards the upstream side in the sheet conveying direction by a force T. At this time, the end portion of the belt at the side of the correcting mechanism110R pushes the movable member111towards the upstream side in the sheet conveying direction. As shown inFIG.25, the pushed movable member111tries to rotate around a contact point P between the recess112band the protrusion111bin the direction of arrow W and starts inclining. When the force T exceeds a force Tlimit, at which the movable member111is positionally displaced, the movable member111is inclined, as a result of which a hatched portion Y of the movable member111is positionally displaced toward the upstream side in the sheet conveying direction. As inFIG.24A, the belt9is out of alignment with an axis of rotation (alternate long and short dashed lines) of the pressure roller. Therefore, in order to prevent the movable member from inclining, an inclination restricting mechanism that restricts the inclination of the movable member (inner-surface opposing portion) is provided. More specifically, a first engaging portion111his provided at the end-surface opposing portion111aof the movable member111, and a second engaging portion112his provided at the holding member112. That is, the inclination restricting mechanism includes the first engaging portion provided at the end-surface opposing portion and the second engaging portion that is provided at the holding member and that engages with the first engaging portion.

When the movable member111starts to incline, the first engaging portion111hand the second engaging portion112hcontact each other. As a result, the movable member111is further prevented from inclining. In a state in which the inclination of the movable member is restricted as a result of contact of the first engaging portion111hand the second engaging portion112hwith each other, the protrusion111bof the movable member and the recess (guide)112bof the holding member contact each other at the point P, which is a rotational center of the movable member in the direction of arrow W. However, in the direction of arrow W, at other portions (that is, portions near a point Q inFIG.25), the protrusion and the recess are separated from each other. According to an experiment, the inclination restricting mechanism makes it possible to increase the force TLimit, at which the movable member is positionally displaced when the portion Y of the movable member is pushed towards the upstream side in the conveying direction, by a factor of 1.8. Although, in the sixth embodiment, contact surfaces of the two engaging portions are shaped so as to be parallel to the sheet conveying direction, the contact surfaces may be shaped so as to be inclined with respect to the conveying direction. This makes it possible to continue maintaining the alignment of the belt9without inclining the movable member111, and to continue restricting lateral shift of the belt while reducing stress that is applied to the end surface of the belt.

In the embodiment, it is possible to provide advantages when, as a result of conveying the sheet S that is displaced from its normal position in a width direction, the rotary force F that is transmitted to the belt9from the pressure roller3becomes nonuniform in the longitudinal direction and the force T that tries to move an end surface of the belt at the side that is not laterally shifted towards the upstream side in the sheet conveying direction is generated.

The first engaging portion and the second engaging portion may have shapes shown inFIG.26. InFIG.26, a rib-shaped portion (second engaging portion)212his provided at a holding member212of a correcting mechanism210R, a protrusion (first engaging portion)211his provided at a movable member211, and the protrusion211his held by the rib-shaped portion212h. Even such shapes make it possible to reliably prevent the movable member at the side where the belt is not laterally shifted from being positionally displaced towards the upstream side in the sheet conveying direction by the pushing force from the belt. Since, inFIG.26, reference numerals211b,212b, and212drepresent parts that have the same functions as those of the protrusion111b, the recess112b, and the stopper112dshown inFIG.22, they are not described.

Seventh Embodiment

Next, a seventh embodiment of the present invention is described with reference toFIGS.27to29. Descriptions that are the same as those of the sixth embodiment are not given. Although, in the sixth embodiment, the holding member restricts the inclination of the movable member, parts other than the holding member restrict the inclination of the movable member in the seventh embodiment.

In an example shown inFIG.27, a protrusion (first engaging portion)311his provided at a movable member311, and a groove (second engaging portion)308hwith which the protrusion311hengages is provided at a pressure stay308. InFIG.27, when a belt9is laterally shifted towards a correcting mechanism310L that is disposed opposite to a correcting mechanism310R, the movable member311in the correcting mechanism310R is urged by an urging member14, and collides with the pressure stay308, so that the protrusion311hand the groove308hengage each other.

As in the sixth embodiment, when a force T that causes the belt9and a pressure roller3to be out of alignment acts, the movable member311in the correcting mechanism310R is pushed towards an upstream side in a sheet conveying direction. The pushed movable member311tries to incline in the direction of arrow W around a contact point P between a slide rib-shaped portion311band a guide312b. Here, the protrusion311hof the movable member311and the groove308hof the pressure stay308engage each other to prevent the movable member311from inclining.

In an example shown inFIG.28, a protrusion (second engaging portion)408his provided at a side surface of a pressure stay408at a downstream side in a sheet conveying direction, and the protrusion408his caused to contact an abutting portion (first engaging portion)411hof the movable member411to prevent the movable member411from inclining.

In an example shown inFIG.29, a protrusion (second engaging portion)506his provided at a side surface of a heater holder506(which holds a ceramic heater505) at a downstream side in a sheet conveying direction, and the protrusion506his caused to contact an abutting portion (first engaging portion)511hof a movable member511to prevent the movable member511from inclining. Since, inFIGS.27to29, reference numerals311b,312b,411b,412b,511b, and512brepresent parts that have the same functions as those of the protrusion111band the recess112bshown inFIG.22, they are not described.

Eighth Embodiment

Next, correcting mechanisms610R and610L that correct the inclination of a belt9according to an eighth embodiment are described with reference toFIGS.30A to34B. The shape of the correcting mechanism610R and the shape of the correcting mechanism610L are substantially axially symmetrical with reference to a conveyance reference X of a recording material P. Therefore, the correcting mechanisms610R and610L are described by primarily describing the correcting mechanism610R and partly describing the correcting mechanism610L.

FIG.30Ais a perspective view of the correcting mechanism610L.FIG.30Bis a sectional view of the correcting mechanism610L when seen from a downstream side in a recording material conveying direction.FIG.31Ais a perspective view of a movable member611(described below).FIGS.31B to31Dare a perspective view, a front view, and a sectional view taken along line XXXID of a holding member612that holds the movable member611. Further,FIG.32illustrates the correcting mechanism610L when seen from the direction of arrow XXXII inFIG.30B.FIGS.33A to34Beach illustrate a mechanism that corrects the orientation of the belt by the correcting mechanisms.

The correcting mechanism610L includes a movable member (restricting member)611, a holding member612that holds the movable member611, and compression springs (urging members)614(614a,614b) that urge the movable member611. As described above, the holding member612is fitted to a U-shaped recess of a frame13of a fixing device. This causes the position of the holding member612in a longitudinal direction of a heater and the position of the holding member612in the recording material conveying direction to be substantially determined. Since the holding member612is urged towards a roller3by a spring7, the holding member612is in a substantially secured state.

The movable member611is a part that is movably engaged with the holding member612. The movable member611is in contact with a cutaway portion provided at an end portion of a stay8in a longitudinal direction. A slight gap is provided between the holding member612and a top portion of the movable member611. As shown inFIG.30A, the movable member611has an end-surface opposing portion611athat opposes an end surface of the belt9. When the belt9is laterally shifted in a generatrix direction thereof, the end surface of the belt9collides with the end-surface opposing portion611a. The movable member611has an inner-surface opposing portion611cthat opposes an inner surface of the end portion of the belt9. A slight clearance is provided between the inner surface of the belt9and the inner-surface opposing portion611c. The inner-surface opposing portion611chas the function of guiding the inner surface of the belt9when the belt rotates.

As shown inFIG.31A, the movable member611has a protrusion611bextending obliquely with respect to the longitudinal direction of the heater. As shown inFIGS.31B to31D, the holding member612has a recess (guide)612bextending obliquely with respect to the longitudinal direction of the heater. When the movable member611and the holding member612are combined, the protrusion611bof the movable member611is fitted in the recess612bof the holding member612. By virtue of this structure, the movable member611is slidably held along the recess612bof the holding member612.

Reference numerals614aand614bdenote compression springs (urging members) that urge the movable member611away from a seating surface612aof the holding member612(that is, urge the movable member611towards an end surface of the belt). There are a plurality of urging members in the embodiment. Coil springs are used as the urging members. While the movable member611is not pushed by the belt9, the coil springs614aand614bare disposed at an area that is outside of an area CA (seeFIG.32) of the movable member611with which the end surface of the belt. Although described later, the coil springs are disposed so that at least positions614X at the centers of the coil springs are positioned outside of the area CA. The spring holding seat612aon which the coil springs are mounted are provided at the holding member612.

Next, the operations of the correcting mechanisms610are described with reference toFIGS.32to34B.FIGS.32and33Aeach illustrate a state of a correcting mechanism in which the end surface of the belt9is not in contact with the end-surface opposing portion611a. When the belt9is rotated by following the rotation of the roller3, the belt9contacts the inner-surface opposing portion611cof the movable member611in an area that is disposed upstream of the heater5in a rotation direction of the belt. In contrast, in an area that is disposed downstream of the heater5in the rotation direction of the belt, the belt9is separated from the inner-surface opposing portion611cof the movable member611.

When the end surface of the belt9is not in contact with the end-surface opposing portion611a, the movable member611that is urged by the springs614aand614bis positioned at a farthest location from the spring holding seat612ain the holding member612. At this time, the movable member611collides with a stopper (not shown) provided at the holding member612, so that, even if the movable member611is urged by the springs614aand614b, the movement of the movable member611is restricted, as a result of which the movable member611is positioned.

As shown inFIG.33A, when the end surface of the belt9is not in contact with the end-surface opposing portion611a, the distance between the end surface of the belt9and the end-surface opposing portion611aof the movable member611is D1. The distance from the holding seat612aof the holding member612to the end-surface opposing portion611aof the movable member611is D2.

FIG.33Bshows a state in which the end surface of the belt9contacts the end-surface opposing portion611aas a result of lateral shift of the belt9in the direction of arrow M1 and the belt9pushes the movable member611in the direction of arrow M1 against the urging force of the springs614aand614b.

When, for example, the belt9is laterally shifted towards the movable member611as a result of, for example, the roller3and the belt9being out of alignment with each other, the end surface of the belt9comes into contact with the movable member611. When the belt9is laterally shifted further, the belt9pushes the movable member in the direction of arrow M1 against the urging force of the springs614aand614b, so that the movable member611moves by making use of a force of lateral shift of the belt.

Since the protrusion611bof the movable member611moves along the recess612bof the holding member612, the movable member611moves in the direction of arrow M2. When the protrusion611bcollides with an end portion of the recess612b, the movable member611stops moving. As this time, as shown inFIG.33B, the distance from the holding seat612aof the holding member612to the end-surface opposing portion611aof the movable member611is D3 (<D2). Compared to the state inFIG.33A, the movable member611is moved through a distance D4 towards an upstream side in the recording material conveying direction S.

As mentioned above, when the belt9is rotating, the inner surface of the belt9is in contact with the inner-surface opposing portion611cof the movable member611. Therefore, when, as shown inFIG.33B, the movable member611is moved towards the upstream side in the recording material conveying direction S, the inner-surface opposing portion611cpushes the inner surface of the belt9, so that the end portion of the belt at the side of the correcting mechanism610L moves towards the upstream side in the recording material conveying direction S. In contrast, since the correcting mechanism610R that is positioned opposite to the correcting mechanism610L in the longitudinal direction of the heater is not pushed by the end surface of the belt9, the movable member of the correcting mechanism610R does not move.

When the movement direction of lateral shift of the belt9is in the opposite direction, that is, when the belt collides with the correcting mechanism610R, only the movable member in the correcting mechanism610R moves towards the upstream side in the recording material conveying direction S. This movement causes the end portion of the belt at the side of the correcting mechanism610R to move towards the upstream side in the recording material conveying direction S.

In this way, when the belt9is laterally shifted in the longitudinal direction of the heater (that is, the generatrix direction of the belt), and collides with one of the correcting mechanisms610R and610L, only the end portion of the belt9on the downstream side in a lateral shift direction receives a force towards the upstream side in the recording material conveying direction. Due to this principle, the alignment of the belt9with respect to the roller3is changed, the orientation of the belt is corrected, and the belt moves away from the movable member (that is, in a direction opposite to the direction of arrow M1 shown inFIG.33B), so that the force that is applied to the end surface of the belt9is restricted. This makes it possible to restrict breakage of the belt. As mentioned above, the movable member611is urged by the springs614aand614b. Therefore, when the belt9moves in a direction opposite to the direction of arrow M1 from the state shown inFIG.33B, the movable member611is pushed back to the position shown inFIG.33Aor to a position between the positions shown inFIGS.33A and33B.

Next, the principle of reducing stress that is applied to the end surfaces of the belt9is further described with reference toFIGS.34A and34B.FIGS.34Aand34B each illustrate the heating unit2and the roller3when seen from the side of the belt9.FIG.34Aillustrates a state in which the belt is laterally shifted.FIG.34Billustrates a state in which the orientation of the belt has been corrected.

In general, lateral shift of the belt9in the generatrix direction is caused by the roller3and the belt9being out of alignment with each other.FIG.34Aillustrates a state in which the roller3and the belt9are out of alignment with each other. That is,FIG.34Aillustrates a state in which the end portion of the belt at the side of the correcting mechanism610R is inclined towards the downstream side in the recording material conveying direction S and in which the end portion of the belt at the side of the correcting mechanism610L is inclined towards the upstream side in the recording material conveying direction S. As shown inFIG.34A, a force F is applied to the belt9due to the rotation of the roller3. The force F can be broken down into a force F1 in the generatrix direction of the belt9and a force F2 in a direction that is orthogonal to the generatrix direction. The belt9is laterally shifted towards the correcting mechanism610R by the force F1. When the belt9contacts and pushes the movable member611of the correcting mechanism610R, the movable member611is guided to the holding member612and moves towards the upstream side in the recording material conveying direction S. The movement of the movable member611corrects the orientation of the belt9as shown inFIG.34Bon the basis of the aforementioned principle. Since the roller3and the belt9are no longer out of alignment, the angle between the force F and the generatrix direction of the belt9is changed. As a result, the force F1 is reduced, so that stress that is applied to the end surface of the belt9is also reduced.

As mentioned above, since it is possible to reduce stress that is applied to the end surface of the belt9, it is possible to suppress wear on the end surface of the belt9.

When the belt9is laterally shifted as a result of the belt9and the roller3being out of alignment with each other, the end portion of the belt at the side that has been laterally shifted is inclined downstream in the sheet conveying direction. Thereafter, when the belt9collides with the end-surface opposing portion611aof the movable member611, as shown inFIG.35, the belt9collides with an area of the end-surface opposing portion611aat the upstream side in the sheet conveying direction. When the belt9has collided with the end-surface opposing portion611a, the movable member611is subjected to a force that rotates the protrusion611b(in the direction of arrow RO shown inFIG.35) so as to collide with the recess612bat a point Q with a point P of the protrusion611bserving as a fulcrum. Therefore, the protrusion611bof the movable member and the recess612bof the holding member are jammed, as a result of which the movable member is prevented from moving smoothly.

In contrast, in the embodiment, while the movable member611is not pushed by the belt9, the coil springs614aand614bas a whole are disposed at an area that is outside of the area CA (seeFIG.32) of the movable member611with which the end surface of the belt contacts. Therefore, with respect to a moment in the direction of arrow RO, a force CF of the spring614abecomes an opposing force, and acts to reduce a force that is applied of each of the points P and Q. This allows the movable member611to move smoothly along the recess612bof the holding member612. The coil springs only need to be disposed so that the positions614X of the centers of the coil springs are situated outside of the area CA.

While the belt9is being laterally shifted, the force of the spring614aacts as a force that opposes the moment in the direction of arrow RO. This is because the spring614ais disposed outside of the belt contact area CA at the end-surface opposing portion (that is, towards the upstream side in the sheet conveying direction). The magnitude of the opposing force that is generated as a result of compression of the spring614ais the same as the magnitude of the force for pushing the end-surface opposing portion611athat is generated as a result of lateral shift of the belt9. A distance L2 up to the spring614ais larger than a distance L1 from the fulcrum P to a point where the end-surface opposing portion611acontacts the belt9. Therefore, the force CF effectively acts to cancel the moment in the direction of arrow RO.

If the belt9is inclined in a direction that is opposite to the direction of inclination shown inFIG.35, the spring614bacts similarly to the spring614aand provides an opposing force against a moment in a direction opposite to the direction of arrow RO, so that the movable member611is smoothly guided and moved.

Although, in the embodiment, correcting mechanisms are provided at both opposing ends of the belt, the aforementioned correcting mechanism may be provided at only a side towards which the belt is laterally shifted, with the direction in which the belt is laterally shifted being previously set in one direction. In addition, in the embodiment, the length of the belt is assumed as being less than the span between the two movable members. However, the length of the belt may be about the same as the span between the two movable members, that is, the two ends of the belt may be constantly in contact with the two movable members.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.