IMAGE HEATING APPARATUS

An image heating apparatus includes rotatable members; a roller; a detector; a displacing mechanism configured to displace one longitudinal end of the roller from a first position to a second position depending on an output of the detector so as to return the endless belt to the predetermined zone; and a timer configured to count a time in which the endless belt is out of the predetermined zone from displacement of the roller to the second position by the displacing mechanism. When the time counted by the timer is a predetermined time, the displacing mechanism displaces the one longitudinal end of the roller to a third position remoter from the first position than the second position so as to return the endless belt is returned to the predetermined zone.

FIELD OF THE INVENTION AND RELATED ART

The present invention relates to an image heating apparatus for heating a toner image on a recording material. The image heating apparatus is usable in, e.g., an image forming apparatus such as a copying machine, a printer, a facsimile machine or a multi-function machine having a plurality of functions of these machines.

Japanese Laid-Open Patent Application (JP-A) Hei 5-238614 discloses a fixing device using an endless belt. In such an endless belt using the endless belt, a countermeasure against a phenomenon that the endless belt shifts in a widthwise direction thereof is required.

Therefore, in the device described in JP-A Hei 5-238614, a constitution in which a roller for stretching the endless belt is inclined relative to another roller and thus the endless belt is swung positively in a predetermined zone is employed.

Specifically, in the device described in JP-A Hei 5-238614, a constitution in which a position of the endless belt with respect to a widthwise direction of the endless belt is detected at a plurality of levels is employed. More specifically, the position of the endless belt is detected using a plurality of sensors provided so that their positions are shifted from each other. Then, depending on the position of the endless belt, an inclination angle of the roller is controlled.

However, in such a constitution, a plurality of sensors are required, so that there is room for improvement.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided an image heating apparatus comprising: a pair of rotatable members at least one of which is an endless belt and configured to form a nip for heating a toner image on a recording material; a roller configured to rotatably support the endless belt; a detector configured to detect that the endless belt is out of a predetermined zone with respect to a widthwise direction thereof; a displacing mechanism configured to displace one longitudinal end of the roller from a first position to a second position depending on an output of the detector so as to return the endless belt to the predetermined zone; and a timer configured to count a time in which the endless belt is out of the predetermined zone from displacement of the roller to the second position by the displacing mechanism, wherein when the time counted by the timer is a predetermined time, the displacing mechanism displaces the one longitudinal end of the roller to a third position remoter from the first position than the second position so as to return the endless belt to the predetermined zone.

DESCRIPTION OF THE EMBODIMENTS

FIG. 2is a schematic illustration of an example of an image forming apparatus1. The image forming apparatus1includes four image forming portions U (UY, UM, UC, UK) for yellow (Y), magenta (M), cyan (C), black (K). At each of the image forming portions U, a photosensitive drum (image bearing member)2electrically charged by a charging roller3is exposed to laser light from a laser scanner4depending on image information inputted from an external host device23into a controller (hereinafter referred to as CPU)10, so that an electrostatic latent image is formed thereon.

The formed electrostatic latent image is developed as a toner image of an associated one of the colors with toner of the associated color by a developing device5. The formed toner images of the respective colors are superposedly transferred onto an intermediary transfer belt8by primary transfer rollers6, so that a full-color toner image is formed.

On the other hand, a recording material (hereinafter referred to as a sheet) S accommodated in a cassette15or16is fed along a feeding path17by a feeding roller pair11, a conveying roller pair12and a registration roller pair18, and is fed to a secondary transfer nip which is a press-contact portion between the intermediary transfer belt8and a secondary transfer roller14. Onto the sheet S fed to the secondary transfer nip, the full-color toner image is secondary-transferred, and the sheet S is passes through a feeding path19and is fed to a fixing device100. The sheet S is heated and pressed by the fixing device100, and the toner image is fixed on the sheet S, and then the sheet S is discharged onto a discharge tray21by a discharging roller20.

The CPU10manages all of device contact operations of the image forming apparatus. An operating portion24for permitting input of various pieces of information into the CPU10includes a display portion where the various pieces of information are displayed.

FIG. 3is a left-side illustration of the fixing device100, andFIG. 4is a cross-sectional illustration of the fixing device100. Here, in this embodiment, a front surface (side) of the fixing device100is a surface in a sheet entrance side, and a rear surface (side) of the fixing device100is a surface in a sheet exit side. Left and right are those when the fixing device100is seen from the front surface, and a left(-hand) side is the front side or one end (portion) side and a right(-hand) side is the rear side or the other end (portion) side. Upper and lower are those with respect to a direction of gravitation. Upstream and downstream are those with respect to a feeding direction (recording material feeding direction) V of the sheet S.

The fixing device100is an image heating apparatus of a twin-belt nip type and an electromagnetic induction heating (IH) type. The fixing device100roughly includes a heating belt unit A, a pressing belt unit B, an IH heater (heating device, induction heating coil)135and a casing140accommodating these members.

The heating belt unit A includes an endless heating belt (endless belt)130as a first rotatable member. Further, the heating belt unit A includes a plurality of supporting rollers (supporting members), for rotatably supporting (stretching) the heating belt130from an inner surface side thereof, including a driving roller131and a tension roller132for imparting belt tension. Further, the heating belt unit A includes a pad stay137formed of, e.g., stainless steel (SUS material).

The IH heater135is a dielectric heating means (induction heating means) as a heating means for heating the heating belt130and includes an induction heating coil (exciting coil) for heating the heating belt130through electromagnetic induction heating.

The heating belt130may appropriately selected from those which generate heat by the IH heater135and which have a heat resistant property.

For example, a belt prepared by coating, e.g., a 300 μm-thick silicone rubber on a magnetic metal layer of a nickel (metal) layer or a stainless steel layer with a thickness of 75 μm, a width of 380 mm and a peripheral length of 200 mm and then by coating a PFA tube as a surface layer.

The driving roller131is, for example, a roller formed by integrally molding a core metal surface layer formed with a solid stainless steel having an outer diameter of 18 mm with a heat-resistant silicone rubber elastic layer. The tension roller132is, for example, a hollow roller formed of stainless steel having an outer diameter of 20 mm and an inner diameter of about 18 mm.

The tension roller132and the driving roller131are disposed in substantially parallel to each other with a predetermined interval in an upstream side and a downstream side with respect to the sheet feeding direction V. The pad stay137is disposed between the tension roller132and the driving roller131in proximity and parallel to the driving roller131. The heating belt130is extended around the driving roller131, the tension roller132and the pad stay137with a predetermined tension (e.g., 200 N).

The pressing belt unit B includes an endless pressing belt (endless belt)120as a second rotatable member. Further, the pressing belt unit B includes a plurality of supporting rollers (supporting members), for rotatably supporting (stretching) the pressing belt120from an inner surface side thereof, including a pressing roller121and a tension roller122for imparting belt tension. Further, the pressing belt unit B includes a pressing pad125formed of, e.g., a rubber. The pressing pad125is covered with a slidable sheet128. Further, the pressing belt unit B includes an oil applying roller (lubricant applying member)126.

The pressing belt120may appropriately be selected from those which resistant property. For example, a belt prepared by coating, e.g., a300pm-thick silicone rubber on a magnetic metal layer of a nickel (metal) layer with a thickness of 50 μm, a width of 380 mm and a peripheral length of 200 mm and then by coating a PFA tube as a surface layer. The pressing roller121is, for example, a roller formed by integrally molding a core metal surface layer formed with a solid stainless steel having an outer diameter of 20 mm. The tension roller122is, for example, a hollow roller formed of stainless steel having an outer diameter of 20 mm and an inner diameter of about 18 mm.

The tension roller122and the pressing roller121are disposed in substantially parallel to each other with a predetermined interval in an upstream side and a downstream side with respect to the sheet feeding direction V. The pressing pad125is disposed between the tension roller122and the pressing roller121in proximity and parallel to the pressing roller121. The oil applying roller126is disposed in parallel to the tension roller122between the pressing pad125and the tension roller122. The pressing belt120is extended around the pressing roller121, the tension roller122, the pressing pad125and the oil applying roller126with a predetermined tension (e.g., 200 N).

The oil applying roller126is, as shown inFIG. 5, supported rotatably by an arm127supported rotatably about a shaft122aof the tension roller122. Further, the arm127is rotationally urged by an urging member (not shown) in a direction in which the oil applying roller126operations an inner surface of the pressing belt120.

The oil applying roller126includes a heat-resistant an aramid felt impregnated with, as a lubricant, a heat-resistant silicone oil of about 1000 CS in viscosity and is constituted so that an oil in a certain amount is supplied (applied) onto the inner surface of the pressing belt120. As a result, a frictional force between the inner surface of the pressing belt120and the slidable sheet128covering the pressing pad125is reduced, so that durability is improved. The heat-resistant silicone oil supplied to the inner surface of the pressing belt120is also applied onto the surfaces of the pressing roller121and the tension roller122via the inner surface of the pressing belt120.

The pressing belt unit B is disposed below the heating belt unit A. Further, the pressing belt unit B is pressed with a predetermined pressing force against the heating unit A by a pressing operation of a pressing mechanism omitted have from description.

In this pressing state, the heating belt130contacting the driving roller131and the pressing belt20contacting the pressing roller121are press-contacted to each other with the predetermined pressing force. The elastic layer of the driving roller131is elastically distorted in a predetermined amount by the press-contact between the belts toward the driving roller121. Further, the heating belt130contacting the pad stay137and the pressing belt120contacting the pressing pad125are press-contacted to each other with a predetermined pressing force (e.g.,400N). As a result, a broad nip (fixing nip) N with respect to the sheet feeding direction V is formed between the heating belt130and the pressing belt120.

Then, a driving force of a driving motor163controlled by the CPU10is transmitted to a driving gear G mounted to a shaft131aof the driving roller131, so that the driving roller131is rotationally driven and the heating belt130is circulated and rotated in the clockwise direction of an arrow inFIG. 4. In order to stably feed the sheet, drive (driving force) is transmitted between the heating belt130and the driving roller131with reliability. Also the pressing belt120in the pressing unit B side is circulated and rotated in the counterclockwise direction of an arrow by the rotation of the heating belt130with the frictional force with the heating belt130at the nip N. The heating belt130is heated by an IH heater135and is raised to a predetermined fixing temperature and thus is temperature-controlled.

In this state, the sheet S carrying an unfixed toner image t is introduced from an entrance side into the fixing device100, and is guided by a guiding member (not shown) to enter the nip N, so that the sheet N is nipped and fed. As a result, the toner image t is thermocompression-fixed as a fixed image. The sheet S comes out of the nip N and discharged from the fixing device100in an exit side.

As described above, in this embodiment, the heating belt130and the pressing belt120form the nip N for heating the toner image t on the sheet (recording material), and at least one of these belts is an endless belt.

In a rotational process of the heating belt130in the heating belt unit A, a phenomenon (belt shift movement) that the heating belt130shifts toward a front side or a rear side with respect to a widthwise direction perpendicular to the sheet feeding direction V generates. Further, in a rotational process of the pressing belt120in the pressing belt unit B, a phenomenon that also the pressing belt120shifts toward the front side or the rear side with respect to the widthwise direction perpendicular to the sheet feeding direction V generates.FIG. 7shows a state in which the pressing belt120in the pressing belt unit B moves in its rotational process so as to shift toward the front side or the rear side with respect to a widthwise direction W perpendicular to the sheet feeding direction V.

In this embodiment, the belt shift movement is stabilized within a predetermined shift range (predetermined zone) by swing-type shift control. The swing-type shift control is a method in which when movement of a belt position from a widthwise central portion by a predetermined amount or more is detected, the tension roller is inclined (tilted) as a steering member in a direction opposite to a belt shift movement direction.

By repeating the swing-type shift control (belt shift movement control: meandering control), the belt periodically moves from one widthwise side (one direction with respect to the widthwise direction) to the other widthwise side (the other direction with respect to the widthwise direction), and therefore, the belt shift movement can be stably controlled. That is, the belt is constituted so as to be capable of reciprocating in the direction W perpendicular to the feeding direction V of the sheet S.

The heating belt unit A and the pressing belt unit B are provided with similar belt shift controls (belt steering mechanisms), and shift control of the heating belt130and shift control of the pressing belt120are similarly effected independently. Therefore, in the following, the belt shift control mechanism in the pressing belt unit B side and the shift control of the pressing belt120will be described as a representative, and the belt shift control mechanism in the heating belt unit A side and the shift control of the heating belt130will be omitted from description.

With reference toFIGS. 3 and 6, a supporting arm154is provided on a side plate of a casing140in the front side. The supporting arm154is supported by a shaft121aof the pressing roller121in the front side via a bearing154aand is rotatable about the shaft121aas a center (supporting point), and extends toward the sheet entrance side. At a free end portion of the supporting arm154, a pin159is provided. At a portion between the bearing154aof the supporting arm154and the pin159, a movable bearing153having a degree of freedom such that the bearing153is engaged and movable along an elongated hole with respect to a longitudinal direction is provided. A shaft122aof the tension roller122in the front side is supported by the bearing153. The bearing153is moved and urged in a direction in which tension is applied to the pressing belt120by a tension spring (urging member)156. The tension of 20 kgf is applied to the pressing belt120by the tension spring156.

On the side plate of the casing140in the front side, a shaft160is provided in the sheet entrance side. A worm wheel (helical gear)157ais rotatably supported by the shaft160. A fork plate (control arm)152including a U-shaped groove153ais provided integrally with the worm wheel (helical gear)157a.Further, the pin159of the supporting arm154is engaged with and supported by the groove152aof the fork plate152. On the side plate of the casing140in the front side, a stepping motor155is provided. A worm157fixed on a rotation shaft of the motor155is engaged with the worm wheel157a.

The stepping motor155is rotationally driven normally or reversely, so that the fork plate152is rotated in an upper (upward) direction or a lower (downward) direction via the worm157and the worm wheel157a.In interrelation with this rotation, the supporting arm154is rotated about the shaft121ain the upper direction or the lower direction. With this rotation, the shaft122aof the tension roller122in the front side is moved in the upper direction or the lower direction with the rear side shaft122as a supporting point. As a result, a degree of inclination of the tension roller122changes, so that belt shift correction is made.

That is, the tension roller122acts as a steering roller for adjusting a degree of meandering of the pressing belt120with respect to the widthwise direction (longitudinal direction) perpendicular to the movement direction of the pressing belt120. Therefore, in the following, the tension roller122is referred to as the steering roller.

The steering roller122, the fork plate (control arm)152, the worm wheel157a,the worm157, the stepping motor155, and the like which are described above constitute a belt reciprocating control means. That is, these members constitute a belt steering mechanism (displacing mechanism for displacing the steering roller122as a supporting member from a reference position to a first position in one longitudinal end (portion) side).

With reference toFIGS. 4 and 5, the casing140is provided with a sensor portion150for detecting a belt end portion position in a lower side of the pressing belt unit B is provided in the neighborhood of an end portion of the fixing device in the front side with respect to the widthwise direction of the pressing belt120perpendicular to the sheet feeding direction V. That is, the sensor portion150is a position detecting means (detector for detecting that the belt is out of a predetermined zone with respect to the widthwise direction of the belt) capable of detecting shift movement of the pressing belt120from one end portion side to the other end portion side with respect to the widthwise direction of the pressing belt120.

The CPU10detects the end portion position of the pressing belt120by the sensor portion150, and effects the belt shift correction by changing the degree of inclination of the steering roller122constituting the belt steering mechanism. As a result, control is effected so that reciprocating movement of the pressing belt120is made between the front side (one end portion side) and the rear side (the other end portion side) with respect to the widthwise direction of the pressing belt120. That is, the belt steering mechanism is controlled so that the pressing belt120makes reciprocating movement in the predetermined zone with respect to the widthwise direction.

Incidentally, an operation of the belt shift control in the front side and a similar operation of the belt shift control in the rear side are performed independently. That is, the sensor portion150is provided at each of end portions of the pressing belt120with respect to the widthwise direction of the pressing belt120, so that execution of operations in a first control mode and a second control mode, which are described later, in one end portion side and operations in the first control mode and the second control mode in the other end portion side are independently made.

The belt shift control is specifically effected by detecting the end portion position of the pressing belt120by the sensor portion150, driving the stepping motor155plural times correspondingly thereto and by rotating the pressing belt120in a state in which the degree of the inclination of the steering roller122. As a result, the shift control of the belt in an axial direction (widthwise direction) of the belt is realized. By changing alignment of the steering roller122, a movement amount of the pressing belt120in the widthwise direction can be controlled.

The sensor portion150is a belt position detecting means for detecting a movement position of the pressing belt120with respect to the axial direction (widthwise direction) of the belt. The sensor portion150is constituted by first and second (two) sensors150aand150b,a sensor flag150cwhich is a flag member, a sensor arm150dand a sensor spring150efor operating the sensor arm150dso as to follow motion of the pressing belt120. The sensor flag150crotates about a supporting point150fis interrelation with the sensor arm150d.

Further, the sensor arm150dwhich is an arm member is urged and contacted to a front-side end surface (edge)120aof the pressing belt120in one end portion side of the pressing belt120with a force of 3 gf. As a result, by a combination of ON/OFF signals of each of the first and second sensors150aand150bas a flag detecting means for optically detecting the sensor flag150cwhich is the flag member rotating depending on the belt position with respect to the widthwise direction of the belt, the position detection of the pressing belt120with respect to the widthwise direction is made.

A relation between the combination of the ON/OFF signals of each of the first sensor150aand the second sensor150band the end surface (edge) position of the pressing belt120at that time is shown inFIG. 8. Incidentally, when light travelling toward each of the first and second sensors150a,150bis blocked by the sensor flag150c,the signal is an OFF signal, and when the light is transmitted to reach the each of the first and second sensors150a,150b,the signal is an ON signal.

Further, a flowchart of the belt reciprocating control is shown inFIG. 1(FIGS. 1A and 1B), and a block diagram of a control system for driving the steering roller122is shown inFIG. 9. Further,FIG. 7shows an operation of the steering roller122for causing the pressing belt120to make the reciprocating movement.FIG. 10shows a positional relation between a position of the pressing belt120with respect to a reciprocating direction and the first and second sensors150a,150b.

The pressing belt120reciprocates between a position where the first sensor150ais ON and the second sensor150bis OFF and a position where the first sensor150ais OFF and the second sensor150bis ON. The reciprocating control is effected so that the pressing belt120exists in a section therebetween (within a predetermined zone). A distance of the section is ±1.5 mm of a center position of the pressing belt120with respect to a rotational axis direction of the pressing belt120.

As shown inFIG. 10, when the pressing belt120moves from the center position toward the front side, the sensor flag150cacts through the sensor arm150dcontacting the pressing belt120.

When the pressing belt120reaches a position of −1.5 mm in the front side, the sensor flag150cis detected by the first sensor150aand the signal of the first sensor150abecomes the OFF signal, so that the steering roller122acts for moving the pressing belt120in the direction toward the rear side.

The movement direction of the pressing belt120is reversed and when the pressing belt120falls within a range of ±1.5 mm, the sensor flag150cmoves from the position where the sensor flag150cis detected by the first sensor150a,so that the signal of this sensor150abecomes the ON signal (also the signal of the second sensor150bis the ON signal).

On the other hand, when the pressing belt120reaches a position of +1.5 mm in the rear side, the sensor flag150cis detected by the first sensor150aand the signal of the second sensor150bbecomes the OFF signal, so that the steering roller122acts for moving the pressing belt120in the direction toward the front side.

The movement direction of the pressing belt120is reversed and when the pressing belt120falls within a range of ±1.5 mm, the sensor flag150cmoves from the position where the sensor flag150cis detected by the second sensor150b,so that the signal of this sensor150bbecomes the ON signal (also the signal of the first sensor150ais the ON signal).

At a position of ±3.5 mm with respect to the reciprocating movement direction of the pressing belt120, an arm wall surface127aof the arm127exists (in both of the front side and the rear side). A position of ±3.0 of the pressing belt120is set as a movement regulating position in order to prevent breakage of the pressing belt120by contact with the arm surface127adue to movement of the pressing belt120in the reciprocating direction.

That is, as shown inFIG. 10, when the pressing belt120reaches the position of −3.0 mm as the movement regulating position, the sensor flag150cmoves to a position where the sensor flag150cis detected by both of the sensors150a,150b.As a result, the signals of the sensors150a,150bbecome the OFF signals. On the other hand, this is also true for the case where the pressing belt120reaches the position of +3.0 mm as the movement regulating position.

Next, with reference toFIG. 1andFIGS. 11 and 12, the belt reciprocating control in this embodiment will be described.FIGS. 11 and 12are timing charts of the signals of the first sensor150aand the second sensor150band an operating angle of the steering roller122.

The fixing device100is driven, and with start of rotation of the heating belt130and the pressing belt120by the driving motor163, the belt reciprocating control is started (S-01, S-02, S-03).

When the pressing belt120shifts toward the end portion side with respect to the widthwise direction, the sensor portion150detects that the pressing belt120reached a reciprocating direction changing position (S-10or S1-23). Then, the CPU10which is a control means executes an operation in the first control mode. The first control mode is a control mode in which the belt steering mechanism is controlled so that the pressing belt120is moved to the other end portion side with a first control amount against the shift movement of the pressing belt120, detected by the sensor portion150, toward one end portion side with respect to the widthwise direction.

Specifically, with the first control amount, the movement direction of the pressing belt120with respect to the widthwise direction is changed to an opposite direction through the steering roller122. That is, through the motor driver155D (FIG. 9), a predetermined driving pulse is outputted to the stepping motor155(S1-17or S1-30). Then, as shown inFIG. 7, by moving the front-side end portion of the steering roller122upward and downward, the steering roller122is inclined relative to the driving roller131by +α° or −β° (S1-18or S1-31).

The pressing belt120changes the reciprocating direction and starts movement. Then, through the first sensor150aand the second sensor150b,the belt position detecting portion150detects that the pressing belt120changes in reciprocating direction and passes (S1-19, S1-32).

Angles α and β of the steering roller122are set to provide an inclination angle at which the reciprocating direction is not changed for maintaining the reciprocating control of the pressing belt120, and in this embodiment, initial values thereof are 2° and −2°, respectively. The angles α and β are variable values depending on a reciprocating time of the pressing belt120(i.e., in interrelation with a movement time).

In the case where the belt shift is detected, by a first means, with the inclination of the steering roller122by +α° or −β° and the rotation of the pressing belt120, the shift of the pressing belt120in the end portion direction is decelerated and is accelerated in the opposite direction. That is, reversal of the belt reciprocating direction is made in this manner (operation ofFIG. 11).

Incidentally, misalignment between members for stretching the pressing belt120generates due to a horizontal state of the image forming apparatus1and tolerances or the like of constituent parts of the fixing device100, so that the reciprocating movement of the pressing belt120becomes non-uniform in some cases. For example, in the case where movement of the pressing belt120in the front side direction (−direction) is fastened the movement in the rear side direction (+direction) is slow, responsiveness when the movement direction of the pressing belt120is changed from the front side direction to the rear side direction lowers.

In such a case, reduction in moving speed of the pressing belt120from the rear side direction to the front side direction by decreasing an absolute value of the angle β is effective. As a result, it becomes possible to reduce a deceleration/acceleration time of the pressing belt120during change in reciprocating direction in the front side, so that responsiveness when the movement direction is changed from the front side direction to the rear side direction can be improved.

FIG. 13shows an example of a relation between a maximum movement position where the pressing belt120moves when the pressing belt120moves to the front side with respect to the reciprocating direction, and α° and β°.

That is, in the case where β° is 2° and α° is 2°, the moving speed of the pressing belt from the rear side to the front side is fast. For that reason, when the angle of the steering roller122is changed from +2° to −2°, the deceleration takes time due to a factor such as slip with the steering roller122, so that the pressing belt120moves to a position of about −2.7 mm in the front side.

On the other hand, in the case where β° is 1° and α° is 1°, the moving speed of the pressing belt from the rear side to the front side is slow. For that reason, when the angle of the steering roller122is changed from +1° to −2°, responsiveness of the reciprocating direction change of the pressing belt120to the angle change of the steering roller122is improved. As a result, the pressing belt120moves to a position of about −1.8 mm in the front side, so that reversal of the reciprocating operation can be completed.

That is, the inclination angle of the steering roller122is decreased, so that a reciprocating movement speed of the pressing belt120can be reduced. As a result, a degree of displacement of the sheet S, in the reciprocating movement direction of the pressing belt120, nipped and fed at the nip N formed by the heating belt130and the pressing belt120can be reduced, so that stabilization of sheet behavior can be realized.

In this embodiment, control of changing the angles α, β is effected on the basis of a time from a state of S1-10to a state of S1-23of the pressing belt120inFIG. 1and a time required for the reciprocating movement from the state of S1-23to the state of S1-10.

Specifically, when the time of movement of the pressing belt120from the state of S1-10to the state of S1-23is less than 60 sec, the angle α changes by −0.1°. Further, when the time of movement of the pressing belt120from the state of S1-10to the state of S1-23is not less than 60 sec, the angle α increases by 0.1°. At this time, in order to prevent generation of abnormal reciprocating control and a change in reciprocating direction of the pressing belt120, the change in angle α is limited so that the value of α is 1.0° to 2.0°.

Similarly, when the time of movement of the pressing belt120from the state of S1-23to the state of S1-10is less than 60 sec, the angle β decreases by 0.1°. Further, when the time of movement of the pressing belt120from the state of S1-23to the state of S1-10is not less than 60 sec, the angle β increases by 0.1°. At this time, also the change in angle β is limited so that the value of β is 1.0° to 2.0°.

The above-described changes in angles α, β are made for not only improving control stability of the pressing belt120by adjusting a reciprocating speed reducing amount of the pressing belt120but also reducing the influence of the movement of the pressing belt120in the reciprocating direction on the sheet behavior.

Incidentally, in the case where the reciprocating movement of the pressing belt120is liable to be subjected to disturbance, there arises a need to ensure a large inclination angle of the steering roller122. In the fixing device100described in this embodiment, silicone oil application is made between the inner surface of the pressing belt120urged by the pressing pad125and the slidable sheet128. For that reason, when the belt reciprocating direction is reversed, sliding generates between the pressing belt120and the roller stretching the pressing belt120, so that responsiveness to reversal of the belt reciprocating direction lowers.

Further, an amount of the silicone oil supplied from the oil applying roller126to the inner surface of the pressing belt120decreases with an increase in contact time, and therefore a lubrication state changes between a brand-new state and a durable state. When the oil amount decreases, a degree of the sliding between the pressing belt120and the roller stretching the pressing belt120decreases, so that the responsiveness to the reversal of the belt reciprocating direction is improved.

That is, the moving speed of the pressing belt120in the reciprocating direction increases and the degree of the displacement of the sheet S in the belt reciprocating direction is increased, so that the behavior lowers. That is, the inclination angle of the steering roller122determined in view of a state in which the responsiveness to the reciprocating direction change of the belt with an ambient initial oil amount tends to gradually lower the sheet behavior due to durable use.

According to the belt reciprocating control type described in this embodiment, even in a system in which the above-described lubrication state changes, it becomes possible to not only improve the control stability of the pressing belt120but also reduce the degree of the influence of the belt reciprocating control on the sheet behavior.

Incidentally, due to the above-described change in amount of the oil supplied from the oil applying roller126and unexpected disturbance, the case where the reciprocating movement of the pressing belt120abruptly becomes non-uniform is assumed. That is, there is a possibility that the pressing belt120moves toward the widthwise end portion side and reaches the reciprocating direction changing position and the reciprocating direction change of the pressing belt120is not made in a state in which the steering roller122is inclined by +α° or −β°.

In the case where detection that the pressing belt120is not changed in reciprocating direction in a predetermined time of X sec is made (S1-19or S1-32), the CPU10as the control means executes an operation in the second control mode. The second control mode is a control mode in which the belt steering mechanism is controlled in the following manner when the sensor portion150detects that the movement of the pressing belt120toward the other end portion side is not made. That is, the belt steering mechanism is controlled so that the pressing belt120is moved toward the other end portion side by changing the control amount to a second control amount larger than the first control amount. The CPU10further inclines the steering roller122with the second control amount.

Specifically, the CPU10includes a timer for counting a time in which the pressing belt120is in a state of being out of the predetermined zone from the displacement of the steering roller122to the first position by the steering mechanism (displacing mechanism). Further, when the time counted by the timer is a predetermined time, the CPU10controls the steering mechanism so that the pressing belt120is returned into the predetermined zone. That is, the CPU10controls the steering mechanism so that the steering roller122is displaced to a second position move spaced from a longitudinal one end portion side position of the steering roller122than the first position is.

That is, when detection times t of the first sensor150aand the second sensor150breach a predetermined time X (sec), through the motor driver155D (FIG. 9), a predetermined driving pulse is outputted to the stepping motor155(S1-12or S1-25). Then, the steering roller122is inclined relative to the driving roller131by +γ° or −η° (S1-13or S1-26).

γ° and η° are values determined in view of angles at which the reciprocating direction of the pressing belt120can be changed against variations of parts and disturbance and the like. In this embodiment, these values are calculated by the CPU10so that γ° is (2−α)° and γ° is (2−β)°.

X(sec) which is a time in which the degree of the change in belt reciprocating direction is a value determined by the moving speed in the belt reciprocating direction due to the rotational speed and the like of the pressing belt120. It is suitable that a time in which the belt is not out of a belt traveling region and the belt end portion does not contact external parts is selected, and in this embodiment, X(sec) is 5 sec.

Similarly, after the reversal of the belt reciprocating direction is made by the first means with the inclination of the steering roller122by +α° or −β° and the rotation of the pressing belt120, there is a possibility that the belt reciprocating movement direction is changed by abrupt disturbance.

The pressing belt120changes its reciprocating direction and starts movement thereof and when passing of the pressing belt120after the change in reciprocating direction is detected in the state S1-19or the state S1-32, whether the pressing belt120passed through which one of the front side and the rear side is recorded by the CPU10(S1-15or S1-28). As a method of recording, for example, a method in which the recording is made by setting the flag in the direction in which the pressing belt120passed and when detection that movement of the pressing belt120in the opposite direction is made (S1-16or S1-29), the flag is reset is suitable.

After the pressing belt120moves toward the widthwise end portion side and detection that the pressing belt120reached the reciprocating direction changing position in the state of S-10or the state of S1-23, the above-described direction in which the belt last passed is checked (S1-11or S1-24).

In the case where the last belt passing direction (front-side direction) is the same as the detected belt movement direction (the case where the belt last passed through the front side in the state of S1-10or the belt last passed through the rear side in the state of S1-23), the CPU10as the control means executes the operation in the second control mode.

Specifically, with the second control amount, the steering roller122is further inclined. That is, through the motor driver155D (FIG. 9), a predetermined driving pulse is outputted to the stepping motor155(S1-12or S1-25). Then, the steering roller122is inclined relative to the driving roller131by +γ° or −η° (S1-13or S1-26). γ° and η° are determined similarly as in the above-described case where the reciprocating direction is not changed.

In the case where the thus-controlled pressing belt120is shifted toward the other end portion side while keeping the inclination angle at α° or β° or at γ° or η°, the steering roller122is subjected to the above-described operation similarly. That is, first, in the operation in the first control mode, the belt is reversed with the inclination angle of α° or β°, and when the belt reversing operation is not performed, the inclination angle is changed to +γ° or −η° in the operation in the second control mode.

As a result, a steering amount for the belt reciprocating control is reduced, so that not only stable belt reciprocating control and stabilization of recording material behavior but also a decrease in steering amount are realized and thus it becomes possible to realize a decrease in a degree of inconvenience of the belt reciprocating control caused by unexpected disturbance.

Therefore, an object to be controlled is not limited to the angle, but is also applied to an angle changing means such as an input pulse into the stepping motor155for changing the angles α°, β° by operating the steering roller122, and thus is not limited to the contents described in this embodiment.

Further, the reciprocating direction change of the pressing belt120is discriminated using a detection time by the detecting means150, so that the change in reciprocating movement behavior of the pressing belt120can be detected by a minimum detecting means. That is, an end portion detecting region (region from −1.5 mm to −3.0 mm in the front side) of the pressing belt120are monitored using a plurality of detecting means. As a result, in a type in which the change in reciprocating behavior of the pressing belt120is detected by the movement amount, the number of detecting means can be reduced, so that it becomes possible to reduce a cost and suppress an upsizing of the image heating apparatus.

Incidentally, in a state in which the reciprocating control is disabled, when the end (edge) surface of the pressing belt120reaches a position of ±3 mm from the center position, both of the first sensor150aand the second sensor150bare turned off (S1-04, S1-07, S1-20). At this time, the CPU10of the image forming apparatus discriminates that abnormality generates, and stops an rotation operation of the belt (S1-05, S1-08, S1-21) and stops the image heating apparatus (S1-06, S1-09, S1-22).

An execution relationship between the above-described operations in the first control mode and the second control mode are summarized as follows.

The tension roller (steering roller)122which is at least one of belt stretching means is inclined with a first inclination angle during execution of the operation in the first control mode. The tension roller122is inclined with a second inclination angle not less than the first inclination angle during the operation in the second control mode.

The CPU10at the control means executes the operation in the first control mode when the shift movement of the pressing belt120is detected by the sensor portion150. Further, the CPU10executes the operation in the second control mode when the pressing belt120moving toward the other end portion side does not pass in a predetermined time.

The CPU10executes the operation in the first control mode when the shift movement of the pressing belt120is detected by the sensor portion150. Further, after the passing of the pressing belt120moving toward the other end portion side is detected by the sensor portion150, when the shift of the pressing belt120in the same direction toward the end portion side is detected again, the CPU10executes the operation in the second control mode.

The CPU10sets the first inclination angle in interrelation with a movement time of the pressing belt120from one end portion side toward the other end portion side.

The sensor portion150is provided at each of the end portions with respect to the widthwise direction of the pressing belt120, and the execution of the operations in the first and second control modes in one end portion side and the execution of the operations in the first and second control modes in the other end portion side are made independently.

Although the description of the belt shift control mechanism in the heating belt unit A side and the shift control of the heating belt130will be omitted, these are similar to the above-described cases of the belt shift control mechanism in the pressing belt unit B side and the belt shift control. InFIGS. 3 and 4, a portion150A corresponds to the sensor portion150in the pressing belt unit B side.

Further, members154A,153A,156A and160A correspond to the supporting arm154, the bearing153, the tension spring156and the shaft160, respectively, in the pressing belt unit B side. Further, members152A,157A and155A correspond to the fork plate152, the worm157and the stepping motor155, respectively, in the pressing belt unit B side.

According to the fixing device in this embodiment, by decreasing the steering amount for the belt reciprocating control, it is possible to realize stable belt reciprocating control and stabilization of sheet behavior.

Modified Embodiments

(1) As described above, a preferred embodiment of the present invention was described, but the present invention is not limited thereto. Various modifications can be made within an equivalent range. For example, the present invention is applied to a constitution in which a belt-shaped member in the fixing device is subjected to the reciprocating control with a substantially predetermined range and is particularly applied to a constitution in which after the reciprocating direction is changed, the reciprocating movement speed can be reduced.

(2) Further, in the above-described embodiment, the pressing belt120was described, but the present invention is also applicable to the heating belt130. That is, the present invention is applicable to the case where at least one of a first rotatable member and a second rotatable member which constitute the nip N includes the endless belt.

(3) In the above-described embodiment, as the image heating apparatus, the fixing device for heating and fixing the unfixed toner image t formed on the sheet (recording material) was described as an example, but the present invention is not limited thereto. The present invention is also applicable to an apparatus for increasing a gloss (glossiness) of an image by reheating a toner image fixed or temporarily fixed on 39-40 the sheet S.

(4) The image forming apparatus is not limited to the image forming apparatus for forming the full-color image as in the above-described embodiment, but may also be an image forming apparatus for forming a monochromatic image. Further, the image forming apparatus can be realized, in various uses, as apparatuses, such as a copying machine, a facsimile machine, a multi-function machine having a plurality of functions of these machines by adding thereto necessary devices, equipment and casing structure.

This application claims the benefit of Japanese Patent Application No. 2015-234634 filed on Dec. 1, 2015, which is hereby incorporated by reference herein in its entirety.