JP4975325B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to belt misalignment control and reliability improvement in an image forming apparatus (monochrome or color) such as a copying machine, a facsimile machine, and a laser beam printer having a transfer belt unit.

  In a conventional image forming apparatus, particularly a color image forming apparatus, each color toner image sequentially formed on a photosensitive drum is superimposed and transferred onto a transfer belt unit (referred to as an intermediate transfer belt in the present specification), and the color thereof. An image having a configuration in which an image is collectively transferred to a transfer material (hereinafter referred to as transfer paper) is used.

In these image forming apparatuses, it is necessary to control the position of the intermediate transfer belt in the main scanning direction (belt shift control) in order to prevent mechanical destruction of the belt. These belt shift controls are based on the conventional method of detecting the position of the end of the belt with a sensor and controlling the position of the belt, or by installing a shift stop guide at the belt opening and contacting the roller end. There are a method for controlling the position of the belt and a means for mechanically regulating the belt end.
JP-A-10-146993

  Of these, in the case of controlling with a sensor, when the position of the belt reaches a certain point, the shifting direction is changed. That is, control is performed so that the belt moves in the opposite direction after moving in one direction. As a result, the amplitude is controlled between two points on both sides. This leads to main-scan color misregistration that occurs during image formation.

  Further, in the case of a contact type sensor, since the belt rotates, there is a problem that the belt is damaged by sliding of the contact portion, and the life of the belt is reduced.

  In the case of an optical sensor, if the sensor surface malfunctions due to contamination, etc., the belt offset control malfunctions, the belt cannot be controlled sufficiently, and an edge shift occurs, resulting in belt breakage. There is.

  In addition, when a detent member is used, there is a case where the belt end edge portion is mechanically broken by a stress applied by the detent member. In addition, when the belt edge is stressed, the belt surface is deformed, and in order to control the belt alignment on the belt surface, noise is generated in the output of the sensor that reads the toner image of each color. It may become impossible.

  In view of the above-described conventional problems, the present invention has an object to extend the life of the belt and control the main scanning movement of the belt by reducing the load applied to the belt detent guide.

An image forming apparatus according to claim 1 of the present invention is an image forming apparatus using an electrophotographic process.
An intermediate transfer member which is an image carrier comprising an endless belt;
A plurality of rollers for stretching the endless belt;
A belt edge regulating member for regulating the edge portions of the endless belt forming the intermediate transfer member,
An optical sensor for detecting irregularities on the surface of the endless belt generated by stress caused by contact of the endless belt end with the belt end edge regulating member ;
Comprising means for varying the deviation direction of the endless belt by a signal of the optical sensor which detects the irregularities on the endless belt surface,
The optical sensor is provided in the vicinity of a driving roller that rotationally drives the endless belt among the plurality of rollers .

According to the second aspect of the present invention, in the image forming apparatus of the first aspect,
And said optical sensor, characterized by using a sensor for detecting the toner density on the endless belt.

According to the third aspect of the present invention, in the image forming apparatus of the first aspect,
Wherein as the optical sensor, which comprises using the sensor for positioning the color image on the endless belt.

According to a fourth aspect of the present invention, in the image forming apparatus of any one of the first to third aspects,
Means for varying the deviation direction of the endless belt, before of Kifuku number of rollers, the driving roller is not contacting the cam bearing around rollers brought against the endless belt, and the bearing said cam Rukoto such a mechanism to press urge biased into contact from the opposite side to the cam, to vary the parallelism with respect to the roller by varying the position of the center axis of the Child around rollers by rotating the cam It is characterized by.

According to a fifth aspect of the present invention, in the image forming apparatus according to any one of the first to fourth aspects,
The means for varying the direction in which the endless belt is shifted is such that the unevenness value of the endless belt detected by the optical sensor is the amount of unevenness detected by the optical sensor at an arbitrary position of the endless belt. The reading value of the optical sensor is set to a value derived from the relationship between the stress generated by the amount of deformation of the belt and the means for varying the direction of the endless belt and the strength of the endless belt. the direction wherein the variable to Rukoto.

According to the present invention, it is possible to prevent the belt from being suddenly damaged by providing a detent guide on the belt, and to obtain means for changing the direction of the belt deviation based on the detection information of the optical sensor for detecting the belt deviation. As a result, it is possible to reduce the load applied to the belt detent guide, extend the life of the belt, and have high reliability. As a result, the environmental load can be suppressed.

  The best mode for carrying out the present invention will be described below with reference to the embodiments shown in the drawings.

  FIG. 1 is a conceptual cross-sectional view showing an example of a color image forming apparatus which is an object of the present invention. First, a process for obtaining a color image in this image forming apparatus will be described.

  The color image forming apparatus shown in FIG. 1 includes a transfer belt unit 10 having an intermediate transfer belt 11 and four image stations. Each image station has an image carrier (hereinafter referred to as a photosensitive drum) 20Y, 20C, 20M and 20Bk, respectively, and charging devices 30Y, 30C, 30M, and 30Bk, developing devices 50Y, 50C, 50M, and 50Bk, and cleaning devices 40Y, 40C, 40M, and 40Bk, respectively.

  In the figure, reference numeral 9 denotes a toner bottle for replenishing toner, which is filled with yellow (Y), cyan (C), magenta (M), and black (Bk) toners from the left in the figure. The developing devices 50Y, 50C, 50M, and 50Bk for each color are replenished by a predetermined replenishment amount depending on the route.

  With respect to the operation of this apparatus, the transfer paper 2 is fed from the paper feed cassette 1 by the paper feed roller 3, and when the leading edge of the transfer paper 2 reaches the registration roller pair 4, it is detected by a sensor (not shown). While being taken, the transfer sheet 2 is conveyed to the nip portion between the secondary transfer roller 5 and the intermediate transfer belt 11 by the registration roller pair 4.

  The photosensitive drums 20Y, 20C, 20M, and 20Bk, which are uniformly charged in advance by the charging devices 30Y, 30C, 30M, and 30Bk, are exposed and scanned with laser light by the optical writing device 8, and the photosensitive drums 20Y, 20C, and 20M are obtained. , An electrostatic latent image is formed on 20 Bk. The electrostatic latent images are developed by the developing devices 50Y, 50C, 50M, and 50Bk for the respective colors, and yellow, cyan, magenta, and black toner images are formed on the surfaces of the photosensitive drums 20Y, 20C, 20M, and 20Bk.

  Next, a voltage is applied to the primary transfer rollers 12Y, 12C, 12M, and 12Bk, and the toner on the photosensitive drums 20Y, 20C, 20M, and 20Bk is sequentially transferred onto the intermediate transfer belt 11. At this time, the image forming operation for each color is executed while shifting the timing from the upstream side toward the downstream side so that the toner image is transferred to the same position on the intermediate transfer belt 11.

  The image formed on the intermediate transfer belt 11 is conveyed to the position of the secondary transfer roller 5 and is secondarily transferred to the transfer paper 2. The transfer paper 2 onto which the toner images of the respective colors are transferred is conveyed to the fixing device 6 and thermally fixed, and is discharged by the paper discharge roller 7.

  The residual toner on the photoconductive drums 20Y, 20C, 20M, and 20Bk is cleaned by the respective cleaning devices 40Y, 40C, 40M, and 40Bk, and then the charging device 30Y, in which the bias of the AC component is superimposed and applied to the direct current. 30C, 30M, and 30Bk are charged simultaneously with static elimination to prepare for the next image formation. The residual toner on the intermediate transfer belt 11 is cleaned by the intermediate transfer belt cleaning device 13 to prepare for the next image forming process.

  FIG. 2 is an enlarged view of one of the image stations of the apparatus of FIG. The developing device 50 includes a developing case 55 having an opening, a developing roller 51 disposed so as to face and face the surface of the photosensitive drum 20, a developing blade 52 that regulates the developer on the developing roller 51 to a certain height, The first conveying screw 53 and the second conveying screw 54 are arranged at positions facing the roller 51.

  The cleaning device 40 includes a cleaning case 43 having an opening, a cleaning blade 41 for cleaning residual toner on the photosensitive drum 20, and a waste toner screw 42 for transporting the cleaned waste toner to a waste toner bottle (not shown). Etc.

  The transfer belt unit 10 includes an intermediate transfer belt 11, a primary transfer roller 12 for transferring the toner image on the photosensitive drum 20 to the intermediate transfer belt 11, an intermediate transfer belt case 14 for holding these components, and the like. It is configured.

  In the figure, 30 is a charging roller as a charging device, 31 is a cleaning roller for the charging roller, and L is a laser beam from the optical writing device 8.

  FIG. 3 is a perspective view showing a schematic configuration of the transfer belt unit 10 including the intermediate transfer belt 11 and the like, and FIG. 4 is a perspective view in which the transfer belt 11 as an image carrier is removed from the configuration of FIG. . The intermediate transfer belt 11 is stretched by at least two or more rollers (three rollers including a driving roller 101, a tension roller 103, and a pre-transfer roller 106 in the drawing), and the intermediate transfer belt 11 is stretched. Two side plates 104 and 105 that hold both ends of the rollers 101, 103, and 106 are provided.

  Next, FIG. 5 shows a configuration in which a detent member 150 is provided at the end of an arbitrary belt stretching roller (for example, the pre-transfer roller 106 in the figure) of the transfer belt unit 10. The stopper member 150 is often made of rubber typified by polyurethane or the like, slidable resin such as elastomer, POM, or the like, but is not limited to such a material in the present invention. The material is selected for reasons such as not damaging the intermediate transfer belt 11 or ensuring slidability.

  Further, this slip-preventing member 150 is provided with a sum R (151), and when the end edge portion of the intermediate transfer belt 11 comes into contact, it receives stress from the sum R portion. When a stronger shift force is generated, the intermediate transfer belt 11 moves toward the outside of the roller along the stain R, and accordingly, stress from the stain R portion also increases, and a shift-back force of the intermediate transfer belt 11 is generated. The position of the intermediate transfer belt 11 is determined at a stage where the offset force of the intermediate transfer belt 11 is balanced.

  By the way, in the transfer belt unit 10 having such a configuration, it is impossible to ideally arrange the parallelism of each part, the inclination of parts, and the like in consideration of the variation of each part. Close to it, receive the force as described above, and gradually approach one side. When a certain amount of deviation occurs and the intermediate transfer belt 10 comes into contact with the stopper member 150, stress is generated on the intermediate transfer belt 11, and the intermediate transfer belt 11 is corrugated to be on the surface of the intermediate transfer belt 11. Concavities and convexities are generated on the surface.

  FIG. 6 shows a portion where the unevenness is likely to occur on the intermediate transfer belt 11 (hatching portion indicated by X in the figure) when viewed from various viewpoints. The portion X where the unevenness is likely to occur is the roller 101 and the transfer belt 11. Is likely to occur in a region that begins to separate from the point Y where the contact occurs. In FIG. 6, 162 is a driving motor, 163 is a cam, 164 is a bearing, and 167 is a spring for pressing and biasing the left cam 163 in the drawing.

  FIG. 7 is a partial cross-sectional view showing the configuration of the intermediate transfer belt 11 and the detent guide 143 of this embodiment. In order to prevent the unevenness of the surface of the intermediate transfer belt 11 from being caused by the above-mentioned shift stop member 150, in this embodiment, a shift stop guide 143 is provided in the vicinity of the edge of the intermediate transfer belt 11 made of a material to be described later such as polyimide. It is used as a detent when the roller 101, 103, 106 or the like is rotated and driven (the pre-transfer roller 106 is shown in the figure).

  However, as shown in FIG. 8, when the unevenness guide 11a is generated on the surface of the intermediate transfer belt 11 because the stopper guide 143 does not function well, the surface of the belt is detected by a sensor.

  A sensor for detecting the surface of the intermediate transfer belt 11 is an example of a reflective sensor 171 that uses a light emitting element 172 and a light receiving element 173 as an optical sensor (FIG. 9: However, this sensor is merely an example. The present invention is not limited to this sense and can be used in various ways.) This sensor is configured to directly detect reflected light, and elements are arranged so that the input angle θ and the reflection angle θ are equal to the object. In this case, when the deflection due to the unevenness 10a occurs in the intermediate transfer belt 11, the input angle θ1 and the reflection angle θ2 change as shown in FIG. 9B, and the amount of light obtained by the light receiving element 173 decreases (illustrated). In the example, it is drawn so that it is no longer incident).

  Therefore, if this phenomenon is detected, it is possible to determine the presence or absence of the unevenness 11a of the intermediate transfer belt 11 and the change thereof, and to determine the state of the force applied to the detent guide 143. In FIG. 9C, the intermediate transfer belt 11 is shifted with time, and the above-described case where the shift guide 143 comes into contact with the pre-transfer roller 106 and the surface of the intermediate transfer belt 11 is gradually bent is described above. The output waveform of such a sensor is converted into a voltage and monitored. In this case, in the portion A in the figure, the intermediate transfer belt 11 maintains a flat surface, and the direct reflected light is strong and the output voltage is maintained high. When the reflected light cannot be picked up (the state shown in FIG. 9B), the output value starts to decrease, and in the illustrated example, it decreases to B in the figure. In the drawing, Z is a point where the detent guide 143 starts to contact the end of the pre-transfer roller 106.

  In this example, it is determined that stress has been applied to the detent guide 143 using data such as the difference, ratio, absolute value, integral value with respect to time constant, differential value, and combinations thereof.

  Next, a method for changing the shifting direction of the intermediate transfer belt 11 based on the information will be described. In the example shown in FIG. 6, the roller 103 is set in the unit so as to rotate with the intermediate transfer belt 11. As described above, the bearing 164 attached to the roller 103 is provided at one end thereof, and the cam 163 that changes the position by rotation of the motor 162 attached to the apparatus main body (not shown) is installed. . When the cam 163 is rotated, when the cam 163 is in contact with the bearing 164 at the position E shown in FIG. 6B, the central axis of the roller 103 is at the position P2 in FIG. When the cam 163 is in contact with the bearing 164 at the position F, the central axis of the roller 103 is at the position P1 in the drawing.

  In addition, as described above, the bearing 164 is pressed and biased by the spring 167 from the opposite side to the cam 163 so that the bearing 164 contacts the cam 163.

The ideal line in which the roller 103 is parallel to the roller 101 is P3 in the figure, and the lines P1 and P2 are often set so as to cross the ideal line P3. By having a mechanism for changing the parallelism of the rollers in this way, it is possible to change the direction of shifting of the intermediate transfer belt 11.

  However, the means for changing the shifting direction of the intermediate transfer belt 11 is not limited to the means shown and described, but for example, the position of the axis for positioning the unit (the axis that is not the axis on which the intermediate transfer belt 11 is stretched) It is also conceivable to change the direction of the intermediate transfer belt 11 by generating a torsional force in the entire unit and changing the direction of the intermediate transfer belt 11.

  In addition, although the optical sensor described above is often used as a sensor that performs detection, instead of providing this sensor separately, the toner pattern transferred onto the intermediate transfer belt 11 is read and the toner density is measured. A toner density sensor that performs control may be used. This makes it possible to obtain a predetermined effect without increasing the number of sensors.

  Note that these sensors are sensors that are generally installed in a color image forming apparatus, and these sensors are often optical sensors, and can satisfy the functions required in the present invention. Further, in the present invention, the following advantages can be obtained by making the detection by the sensor near the drive roller.

  That is, the driving roller needs to increase the frictional force with respect to the belt in order to drive the intermediate transfer belt efficiently. Therefore, the frictional force with the belt is high, and even if thrust is controlled by other rollers, the belt is fixed with its own coefficient of friction, and may not be able to move sufficiently in the thrust direction. If it does so, it will try to control a position of a belt by two parts, As a result, the force of another direction will generate | occur | produce simultaneously with respect to a belt, and an unevenness | corrugation will generate | occur | produce randomly. For this reason, noise becomes larger than the control described above, and the control becomes insufficient. For this reason, by using a roller for controlling the belt shift as a driving roller and installing a sensor in the vicinity of the driving roller, it is possible to stably detect irregularities in a state where these defects are not present or small.

  In addition, when determining the unevenness of the belt and controlling the position of the belt in this way, it is possible to set the belt to come to an arbitrary position by setting the reading value of the sensor so that the detected unevenness amount is obtained. Is possible. This is because the deformation amount of the belt is derived from the relationship between the stress generated between the belt and the belt offset member and the belt strength, and has a certain proportional relationship. Therefore, the set value can be arbitrarily set from a continuous amount. If the belt can be controlled to an arbitrary position in this way, it is possible to reduce color misregistration in the main scanning direction.

  On the other hand, in the case of a sensor that reads the end position of the belt, it is only possible to judge whether there is a belt at an arbitrary reading position, and it is impossible to finely control the position of the belt, Accordingly, the color shift in the main scanning direction is increased.

  Examples of the material for the transfer belt include resins having excellent mechanical properties such as polyimide, polyethersulfone, polyetherketone, or polyetheretherketone. However, it is possible to use materials such as rubber and elastomer. As an example, the polyimide resin has a feature that the deformation of the belt at the time of driving is smaller than that of a conventionally used thermoplastic resin.

  A polyimide resin is generally a polymer synthesized by condensation polymerization using tetracarboxylic dianhydride and diamine or diisocyanate as monomer components. Examples of the tetracarboxylic acid component of the dianhydride include pyromellitic acid, naphthalene-1,4,5,8-tetracarboxylic acid, naphthalene-2,3,6,7-tetracarboxylic acid, 2,3,5, 6-biphenyltetracarboxylic acid, 2,2 ', 3,3'-biphenyltetracarboxylic acid, 3,3', 4,4'-biphenyltetracarboxylic acid, 3,3 ', 4,4'-diphenyl ether tetracarboxylic acid Acid, 3,3 ′, 4,4′-benzophenonetetracarboxylic acid, 3,3 ′, 4,4′-diphenylsulfonetetracarboxylic acid, 3,3 ′, 4,4′-azobenzenetetracarboxylic acid, bis ( 2,3-dicarboxyphenyl) methane, bis (3,4-dicarboxyphenyl) methane, β, β-bis (3,4-dicarboxyphenyl) propane, β, β-bis (3,4 Radical Po hydroxyphenyl) hex-off Oro propane.

  Examples of the diamine component include m-phenyldiamine, p-phenyldiamine, 2,4-diaminotoluene, 2,6-diaminotoluene, 2,4-diaminochlorobenzene, m-xylylenediamine, p-xylylenediamine, 1, 4-diaminonaphthalene, 1,5-diaminonaphthalene, 2,6-diaminonaphthalene, 2,4'-diaminonaphthalebiphenyl, benzidine, 3,3-dimethylbenzidine, 3,3'-dimethoxybenzidine, 3,4 ' -Diaminodiphenyl ether, 4,4'-diaminodiphenyl ether (oxy-p, p'-dianiline; ODA), 4,4'-diaminodiphenyl sulfide, 3,3'-diaminobenzophenone, 4,4'-diaminophenyl sulfone, 4,4'-diaminoazobenzene, 4,4'-diamy Diphenylmethane, beta, beta-bis (4-amine phenyl) propane. The compound which substituted the amino group in the above-mentioned diamine component made into the said isocyanate component with the isocyanate group is mentioned. Commercially available products of these polyimides include, for example, pyromellitic acid-based polyimide (Kapton HA: manufactured by DuPont) having diamine as a diamine component, and 3,3 ′, 4,4′-biphenyltetracarboxylic acid-based polyimide (Upilex S: Ube Industries, Ltd.).

  Conductive agents dispersed in the base material of the belt include conductive carbon materials such as carbon black and graphite, metals or alloys such as aluminum and copper alloys, and tin oxide, zinc oxide, antimony oxide, indium oxide, and titanium. Conductive metal oxides such as potassium oxide, antimony oxide-tin oxide composite oxide (ATO), indium oxide-tin oxide composite oxide (ITO), lithium perchlorate, quaternary ammonium perchlorate, quaternary One kind or two or more kinds of fine powders such as an electrolyte such as ammonium chloride and sodium trifluoromethanesulfonate are used. The metal oxide may be coated with insulating fine particles such as barium sulfate, calcium carbonate, and magnesium silicate.

  Among these conductive agents, carbon black is preferable in terms of price and environmental stability. Furthermore, from the viewpoint of dispersibility, a tin oxide composite oxide (product name: UF) with an average particle size of 0.1 μm manufactured by Mitsui Kinzoku Co., Ltd., a zinc oxide (Pastran Type-II) with 0.3 μm, An average particle size of 1 μm or less, such as a barium sulfate surface with an average particle size of 0.4 μm coated with a tin-based oxide (trade name: Pastorlan Type-IV), 0.2 μm ATO, 0.2 μm ITO, etc. Metal oxides are also preferably used. The conductive metal oxide is preferably surface-treated with one or more silane coupling agents. Since the surface-treated metal oxide is improved in compatibility with the resin constituting the base material, the dispersion thereof becomes uniform and variation in the resistance value of the base material is suppressed.

1 is a conceptual cross-sectional view illustrating an example of a color image forming apparatus that is an object of the present invention. An enlarged view of one of the image stations of the apparatus of FIG. The perspective view which shows schematic structure of the transfer belt unit 10 containing an intermediate transfer belt etc. FIG. 3 is a perspective view in which a transfer belt as an image carrier is removed from the configuration of FIG. The figure which shows the structure by which the detent | locking member was provided in the edge part of the arbitrary belt stretching roller of a transfer belt unit A diagram (A) showing a portion where irregularities are likely to occur on the intermediate transfer belt, and a diagram (B) showing an enlarged portion thereof FIG. 6 is a partial cross-sectional view showing a configuration of an intermediate transfer belt and a detent guide according to an embodiment of the present invention. Diagram showing the unevenness of the intermediate transfer belt due to the stopper guide not functioning well The figure which shows the example of the sensor which detects the surface of an intermediate transfer belt (A, B), and the figure which shows a detection result (C)

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Paper feed cassette 2 Transfer paper 3 Paper feed roller 4 Registration roller pair 5 Secondary transfer roller 6 Fixing device 7 Paper discharge roller 8 Optical writing device 10 Transfer belt unit 11 Intermediate transfer belt 11a Concavity and convexity on intermediate transfer belt 12 Primary transfer Roller 13 Intermediate transfer belt cleaning device 14 Intermediate transfer belt case 20Y, 20C, 20M, 20Bk Photosensitive drum 30Y, 30C, 30M, 30Bk Charging device 31 Cleaning roller for charging roller 40Y, 40C, 40M, 40Bk Cleaning device 41 Cleaning blade 42 Waste toner screw 43 Cleaning case 50Y, 50C, 50M, 50Bk Developing device 51 Developing roller 52 Developing blade 53 First conveying screw 54 Second conveying screw 55 Developing case 101 Driving roller 103 Emissions Deployment rollers 104, 105 side plate 106 pre-transfer roller 143 toward stop guide 150 toward stop member 151 Sumi R
162 Motor for driving 163 Cam 164 Bearing 167 Spring 171 Reflective sensor 172 Light emitting element 173 Light receiving element L Laser beam X Area where unevenness is likely to occur Y Point where roller and transfer belt come in contact Z Detent guide is at end of roller Point where contact began P3 Ideal line where rollers are parallel to each other

P1, P2 Roller axis

Claims (5)

In an image forming apparatus using an electrophotographic process,
An intermediate transfer member which is an image carrier comprising an endless belt;
A plurality of rollers for stretching the endless belt;
A belt edge regulating member for regulating the edge portions of the endless belt forming the intermediate transfer member,
An optical sensor for detecting irregularities on the surface of the endless belt generated by stress caused by contact of the endless belt end with the belt end edge regulating member ;
Comprising means for varying the deviation direction of the endless belt by a signal of the optical sensor which detects the irregularities on the endless belt surface,
An image forming apparatus , wherein the optical sensor is provided in the vicinity of a driving roller that rotationally drives the endless belt among the plurality of rollers . The image forming apparatus according to claim 1 .
And said optical sensor, an image forming apparatus which is characterized by using a sensor for detecting the toner density on the endless belt. The image forming apparatus according to claim 1 .
Wherein as the optical sensor, an image forming apparatus which comprises using a sensor for positioning of the color image on the endless belt. The image forming apparatus according to any one of claims 1 to 3 ,
Means for varying the deviation direction of the endless belt, before of Kifuku number of rollers, the driving roller is not contacting the cam bearing around rollers brought against the endless belt, and the bearing said cam Rukoto such a mechanism to press urge biased into contact from the opposite side to the cam, to vary the parallelism with respect to the roller by varying the position of the center axis of the Child around rollers by rotating the cam An image forming apparatus. The image forming apparatus according to any one of claims 1 to 4,
The means for varying the direction in which the endless belt is shifted is such that the unevenness value of the endless belt detected by the optical sensor is the amount of unevenness detected by the optical sensor at an arbitrary position of the endless belt. The reading value of the optical sensor is set to a value derived from the relationship between the stress generated by the amount of deformation of the belt and the means for varying the direction of the endless belt and the strength of the endless belt. an image forming apparatus comprising a variable to Rukoto direction.