JP2009265444A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which prevents a significant color shift even when using a transfer belt that includes a large coefficient of friction between the transfer belt and a photoreceptor. <P>SOLUTION: The color printer 1 as an image forming apparatus includes: an image forming section which forms a toner image on the photoreceptor rotated; an intermediate transfer belt 11 which is rotated in contact with the photoreceptor and to which a toner image is transferred from the photoreceptor. The color printer 1 includes: a drive motor 33 which rotates the intermediate transfer belt 11; a torque meter 34 which measures a drive torque value of the drive motor 33; and a control section 35 which changes the rotating speed of the drive motor 33 during non image formation, obtains the measurement value of the torque meter 34 at each rotating speed, and determines based on the obtained result the rotating speed indicating an inflection point at which the measurement value steeply changes in relation to the rotating speed of the drive section. During image formation, the drive motor 33 rotates the transfer belt at the rotating speed indicating the determined inflection point. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus capable of forming a color image, such as a printer or a copier. More specifically, the present invention relates to an image forming apparatus having a transfer belt that is in contact with a photoconductor and onto which a toner image is transferred from the surface of the photoconductor.

  2. Description of the Related Art Conventionally, for example, a so-called tandem image forming apparatus has a transfer belt that contacts a photoconductor of each color and receives a toner image of each color. The transfer belt is stretched around, for example, a plurality of rollers and rotated by one of the rotation drives. The toner images of the respective colors formed on the surfaces of the photosensitive members of the respective colors are transferred to the same positions as the toner images of the previous color moving on the transfer belt. Therefore, in order to form a color image without color misregistration, it is required that the toner image of the previous color is opposed to the photoconductor at a desired timing.

  Conventionally, in a belt member such as this transfer belt, the speed is detected by various methods in order to set the traveling speed at least at a position facing the photoreceptor to a desired speed. For example, there is a method for detecting the speed of the belt by forming a mark for speed measurement on the belt or by previously forming a mark on the belt and detecting the position of the mark (for example, patents). Reference 1, Reference 2). Based on the belt speed detected by these methods, a desired speed can be obtained by controlling so as to suppress periodic rotational fluctuations due to eccentricity of the rotating body, unevenness of the belt thickness, and the like.

As a conventional transfer belt, for example, a synthetic resin endless belt or the like is often used. In such a case, at least the outer surface in contact with the photoreceptor is made of a resin having a relatively small friction coefficient. On the other hand, in recent years, it has been studied to use a transfer belt having an elastic layer such as rubber. Such an elastic belt is excellent in adhesion with a recording material such as paper because the belt itself is highly flexible. Therefore, compared to a conventional resin belt, there are advantages that it is less likely to drop out during primary transfer and that transfer efficiency is good even with rough paper during secondary transfer.
Japanese Patent No. 3186610 JP 2001-16883 A

  However, in a transfer belt having an elastic layer, it is not sufficient to control the traveling speed by the various speed detection methods described above. This is because the frictional force with the photosensitive member is larger than that of a conventional resin belt because of the elastic layer. Originally, when there is a relative speed difference between the photoreceptor surface and the transfer belt, slip occurs between them. In a resin belt with a small coefficient of friction, small slips frequently occur, so misalignment due to slip is not a problem. However, in the case of an elastic belt having a large coefficient of friction and high flexibility, slipping hardly occurs. For this reason, the bending of the belt gradually accumulates, and a relatively large slip may occur when the deflection accumulates. When such a large slip occurs, there is a possibility that a color shift occurs at that point.

  Such a large slip is particularly problematic in the case of a transfer belt in contact with a plurality of photoconductors, such as a tandem color printer. This is because there is a case where there is a minute speed difference depending on the photoconductor, and the relative speed difference between the photoconductor and the transfer belt may be slightly different for each photoconductor. For example, when the peripheral speed of one photoconductor is lower than that on the upstream side, the transfer belt tends to bend more upstream than the contact point with the photoconductor. When the bending is eliminated at a stretch by the belt tension, there is a risk of a relatively large color shift. As described above, there is a problem that a large slip is likely to occur particularly when there is unevenness in how the transfer belt bends between the photosensitive members.

  The present invention has been made to solve the problems of the conventional image forming apparatus described above. That is, an object of the present invention is to provide an image forming apparatus that does not cause a large color shift even with a transfer belt having a large coefficient of friction with the photosensitive member.

  An image forming apparatus of the present invention, which has been made for the purpose of solving this problem, is an image forming unit that forms a toner image on a rotationally driven photoconductor, and is rotationally driven while being in contact with the photoconductor. An image forming apparatus having a transfer belt onto which a toner image is transferred, wherein a drive unit that rotationally drives the transfer belt, a torque measurement unit that measures a drive torque value of the drive unit, The measurement value of the torque measurement unit at each rotation speed is obtained by changing the rotation speed. Based on the obtained result, the inflection point at which the measurement value changes most steeply with respect to the rotation speed of the drive unit is indicated. And an inflection point determination unit for determining the rotation speed, and at the time of image formation, the drive unit rotationally drives the transfer belt at a rotation speed indicating the inflection point determined by the inflection point determination unit.

  According to the image forming apparatus of the present invention, the torque measuring unit that measures the driving torque of the driving unit that rotationally drives the transfer belt is provided. Therefore, during non-image formation, the drive torque can be measured by changing the rotation speed of the drive unit. Then, the rotational speed at which the measured drive torque changes most steeply is determined as the inflection point. This inflection point is the rotational speed when the relative speed between the transfer belt and the photosensitive member is at the boundary between positive and negative, and is the rotational speed at which the overall speed difference is the smallest. During image formation, the transfer belt is driven at a rotational speed corresponding to this inflection point, so that the speed difference between the transfer belt and the photosensitive member can be suppressed. Therefore, even with a transfer belt having a large coefficient of friction with the photoreceptor, an image forming apparatus that does not cause a large color shift can be obtained.

Further, in the present invention, it is desirable that the torque measurement unit for determining the inflection point by the inflection point determination unit is measured while rotating the photosensitive member at the same rotation speed as in normal image formation.
In this way, the inflection point can be determined in consideration of the frictional force received from the photoconductor during image formation. Therefore, an inflection point can be obtained in accordance with the state at the time of image formation.

Furthermore, the present invention further includes a belt cleaner that contacts the transfer belt and removes residual toner, and the inflection point determination unit performs measurement of the inflection point by the inflection point determination unit. It is desirable to carry out contact with the same pressure contact force as at the time of formation.
In this way, the inflection point can be determined with the transfer belt receiving substantially the same frictional force as that received from the belt cleaner during image formation. Therefore, an inflection point suitable for image formation can be obtained. If there are other members that are in contact with the transfer belt during image formation, it is desirable to determine the inflection point as much as possible when the image is formed.

Furthermore, in the present invention, the inflection point determination unit, when the acquired drive torque value changes from negative to positive, the rotation speed of the drive unit when the absolute value is the smallest is set as the inflection point, and the acquired drive torque value is obtained. If the torque value does not vary from negative to positive, it is desirable that the rotational speed of the drive unit when the difference in drive torque between adjacent rotational speeds is the largest be the inflection point.
When the drive torque value at each rotational speed measured by changing the rotational speed of the drive unit varies from negative to positive, there is a rotational speed at which the torque value becomes 0 therebetween. This is a point where the driving torque and the frictional force by the photosensitive member are balanced. If the rotational speed very close to this point is taken as the inflection point, the speed difference between the transfer belt and the photosensitive member can be made extremely small. If the measured value does not change from negative to positive, the inflection point at which the measured value changes most steeply with respect to the rotational speed of the drive unit is when the difference in drive torque between adjacent rotational speeds is the largest. The inflection point can be easily determined by using the rotational speed of the drive unit.

Further, in the present invention, the transfer belt is an elastic belt having an elastic layer, and the inflection point determination unit sets the rotation speed of the drive unit when the absolute value of the acquired drive torque value is the smallest as the inflection point. Is desirable.
When the transfer belt is an elastic belt, generally, the frictional force with the photosensitive member is large. For this reason, since there is a rotational speed at which the drive torque value is negative, the inflection point can be appropriately determined in this way.

Furthermore, in the present invention, it is desirable that an image forming unit is provided for each color, and a photoconductor for each color is disposed along the transfer belt.
Such a so-called tandem image forming apparatus is particularly effective.

  According to the image forming apparatus of the present invention, even a transfer belt having a large coefficient of friction with the photosensitive member does not cause a large color shift.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the best mode for embodying the present invention will be described in detail with reference to the accompanying drawings. In this embodiment, the present invention is applied to a color printer having a so-called tandem intermediate transfer belt.

  As shown in FIG. 1, the color printer 1 of this embodiment is a so-called tandem image forming apparatus capable of forming a color image. In the middle of the figure, four color image forming portions are arranged along the intermediate transfer belt 11 in the order of yellow 10Y, magenta 10M, cyan 10C, and black 10K from left to right in the figure. Above the intermediate transfer belt 11, toner storage portions 12 (12Y, 12M, 12C, and 12K) that store toners of the respective colors are provided.

  A detachable paper feed cassette 13 is mounted on the lower portion of the color printer 1 in FIG. Further, on the right side in the figure, a paper transport path 14 is provided from the bottom upward. A paper feed roller 15, a secondary transfer unit 16, a fixing unit 17, and a paper discharge roller 18 are provided in order from the bottom along the paper conveyance path 14. A discharge tray 19 is provided on the upper surface of the color printer 1.

  The image forming units 10Y, 10M, 10C, and 10K for the respective colors include a charging unit 22, an exposing unit 23, a developing unit 24, a primary transfer unit 25, and a cleaner 26 in the clockwise direction in FIG. Have. The exposure unit 23 is provided below the image forming unit for each color, and is common to each color. A belt cleaner 27 is provided in contact with the left end of the intermediate transfer belt 11 in the figure.

  As shown in FIG. 2, the intermediate transfer belt 11 is stretched between a driving roller 31 and a stretching roller 32. Furthermore, this embodiment has a drive motor 33, a torque meter 34, and a control unit 35. The drive motor 33 is for rotating the drive roller 31. The torque meter 34 is attached to the drive shaft of the drive motor 33 and is used for measuring the rotational torque of the drive motor 33. The control unit 35 receives the detection result of the torque meter 34 and controls the drive speed of the drive motor 33. 2 is a view of the intermediate transfer belt 11 and its drive system in FIG. 1 as viewed from above.

  At the time of image formation, the driving roller 31 is driven to rotate counterclockwise in FIG. 1 by the driving motor 33, and the intermediate transfer belt 11 is rotated in the direction indicated by the arrow in the drawing. The color printer 1 of this embodiment can adjust the system speed (traveling speed of the intermediate transfer belt 11) within a range of 100 to 400 mm / sec. For this purpose, a drive motor 33 whose drive torque can be adjusted by the control unit 35 is used. The torque required when only the intermediate transfer belt 11 is driven in a state where the photosensitive member 21 or the like is not in contact with the drive motor 33 is, for example, about 200 to 300 mN · m.

  Next, the operation of each part when a color image is formed by the color printer 1 will be briefly described. At the time of image formation, the intermediate transfer belt 11 and the photosensitive members 21 of the respective colors are rotated as indicated by arrows in FIG. The photoreceptor 21 is uniformly charged by the charging unit 22 and then exposed by the exposure unit 23. As a result, an electrostatic latent image based on the image data is formed on the surface of the photoreceptor 21. Next, toner is supplied to the electrostatic latent image by the developing unit 24, and a toner image is formed on the photoreceptor 21. The toner images of the respective colors are transferred onto the intermediate transfer belt 11 by the primary transfer unit 25 and superimposed. The toner remaining on the photoreceptor 21 even after passing through the primary transfer area is scraped off by the cleaner 26.

  On the other hand, the sheets stored in the sheet feeding cassette 13 are pulled out one by one by the sheet feeding roller 15. Then, the toner image superimposed on the intermediate transfer belt 11 is transferred by the secondary transfer unit 16 to the sheet conveyed through the sheet conveyance path 14. The sheet on which the toner image is transferred is further conveyed upward and reaches the fixing unit 17. In the fixing unit 17, the toner is fixed to the sheet by being heated and pressurized. As a result, the paper on which the image has been formed is discharged to the paper discharge tray 19 by the paper discharge roller 18. Note that the toner remaining on the intermediate transfer belt 11 even after passing through the secondary transfer region is scraped off by the belt cleaner 27.

  The intermediate transfer belt 11 of this embodiment is an elastic belt having a rubber layer. Therefore, it is relatively flexible and has a certain degree of elasticity. The intermediate transfer belt 11 has a coefficient of friction with the surface of the photoreceptor 21 of, for example, μ = 0.7 to 1.0. Therefore, a large frictional force acts between the intermediate transfer belt 11 and the surface of the photosensitive member 21 as compared with a resin belt (for example, μ = about 0.2 to 0.4).

  Further, the intermediate transfer belt 11 of the present embodiment is an elastic belt and is flexible, so that it bends relatively easily. For this reason, if there is a speed difference between the outer surface of the intermediate transfer belt 11 and the outer surface of the photosensitive member 21, or if there is a speed difference between the photosensitive members 21, the belt is likely to bend. Furthermore, if this belt deflection accumulates between the photoreceptors 21, a large slip may occur. A large slip is not preferable because it causes a large color shift. That is, it is preferable that the speed difference between the intermediate transfer belt 11 and the photoreceptor 21 is as small as possible.

  On the other hand, the traveling speed of each portion on the outer surface side of the intermediate transfer belt 11 is not necessarily the same as the peripheral speed of the driving roller 31 due to factors such as belt expansion and contraction. For this reason, even if the rotational speed of the driving roller 31 is set in accordance with the peripheral speed of each photoconductor 21, the contact speed between the photoconductor 21 and the intermediate transfer belt 11 does not always have the same traveling speed. Therefore, the control unit 35 of the present embodiment obtains an inflection point that is a rotational speed at which the relative speed at the contact point between the intermediate transfer belt 11 and each photoconductor 21 is as small as possible overall, and the drive motor is driven at the rotational speed. 33 is driven.

  Further, due to individual differences of the intermediate transfer belt 11 and the like, this inflection point is not uniform even in the same model. Further, the relative speed between the photosensitive member 21 and the intermediate transfer belt 11 depends on the outer diameter of the belt driving roller and the outer diameter of the photosensitive member in addition to the rotational speed of the driving motor 33. These cannot be said to have some variation between the aircraft. Therefore, in this embodiment, the inflection points are determined as follows in each color printer 1.

  In the color printer 1 of the present embodiment, when a new product is manufactured, the relationship between the rotational speed of the drive motor 33 and the drive torque at that time is measured to determine the inflection point. And the process which sets the rotational speed of the obtained inflection point to the rotational speed of the drive motor 33 is performed. That is, when an image is formed with the product, the image is formed while controlling the drive motor 33 with the rotational speed as a target. In order to determine the inflection point, first, the rotational speed of the drive motor 33 is changed little by little, and the drive torque at each rotational speed is measured by the torque meter 34. This is because the drive torque of the drive motor 33 differs depending on the relative speed between the running speed of the intermediate transfer belt 11 and the peripheral speed of the photosensitive member 21.

  For example, the measurement is performed at a plurality of different speeds shifted by about 0.1% in both positive and negative directions at a speed ratio with respect to the photoconductor 21 around the rotational speed at which the peripheral speed of the driving roller 31 is equal to the peripheral speed of each photoconductor 21. To do. The difference from the peripheral speed of the photosensitive member 21 may be measured within a range of about −2% to 2%. In this embodiment, by measuring in this way, the relationship between the rotational speed of the drive motor 33 and the drive torque at that time is acquired as indicated by a solid line A or a broken line B in FIG.

  This measurement is performed in a state where all the members in contact with the intermediate transfer belt 11 are operated in the same manner as in normal image formation. That is, the photosensitive member 21 and the primary transfer unit 25 (in the case of a contact type) are rotated at a rotation speed during normal image formation. Further, the belt cleaner 27 is brought into contact with the intermediate transfer belt 11 with the same pressure contact force as that during normal image formation. Further, it is desirable that the measurement of the driving torque is performed in a state where the moving speed of the intermediate transfer belt 11 is stable. That is, the measurement may be performed after the intermediate transfer belt 11 has traveled two or more times after starting to be driven at the rotational speed at which the torque is to be measured.

  A range where the rotational speed of the drive motor 33 on the right side in FIG. 3 is large is a state where the traveling speed of the intermediate transfer belt 11 is larger than the peripheral speed of each photoconductor 21. In this case, the drive torque of the drive motor 33 overcomes the friction of the entire transfer system of the intermediate transfer belt 11 to drive the intermediate transfer belt 11. Therefore, the driving torque is large. Here, the friction of the entire transfer system of the intermediate transfer belt 11 refers to friction caused by the photosensitive members 21 and other contact members. The friction caused by the other contact members includes, for example, a frictional force between the belt cleaner 27 and a bearing portion of each roller.

  On the other hand, the range in which the rotational speed of the drive motor 33 on the left side in FIG. 3 is small is a state in which the traveling speed of the intermediate transfer belt 11 is smaller than the peripheral speed of each photoconductor 21. In this case, the direction of the frictional force by the photoconductor 21 and the moving direction of the intermediate transfer belt 11 are the same. Therefore, it can be said that the intermediate transfer belt 11 is also driven by each photosensitive member 21 in addition to the drive by the drive motor 33. Accordingly, the drive torque by the drive motor 33 is considerably smaller than when the traveling speed of the intermediate transfer belt 11 is higher than the peripheral speed of each photoconductor 21.

  In particular, when the ratio of the frictional force between the intermediate transfer belt 11 and the photosensitive member 21 is large with respect to the friction of the entire conveying system of the intermediate transfer belt 11, as shown in the example of the solid line A in FIG. May have a negative driving torque (corresponding to a brake). When the intermediate transfer belt 11 is an elastic belt, the frictional force between the photoreceptors 21 is large, and the drive torque often ranges from positive to negative as indicated by the solid line A.

  When the ratio of the frictional force between the photoconductors 21 is not so large, an example of a broken line B in FIG. That is, the driving torque does not become negative, but the driving torque is considerably smaller than the range where the rotational speed of the driving motor 33 is high. In this case, the driving torque does not range from positive to negative. If the surface of the resin belt or elastic belt is slippery, this change will occur.

  There is a range in which the drive torque changes sharply between a range where the rotational speed of the drive motor 33 is large and a range where the rotational speed is small. Within this range, the inflection point of this color printer 1 is obtained. If it is determined from the measurement results that the drive torque is from positive to negative as in the example of the solid line A in FIG. 3, the rotational speed (speed V) of the drive motor 33 at which the drive torque becomes zero. Is the inflection point. In other words, the inflection point of the present invention does not necessarily coincide exactly with the mathematical inflection point.

  Further, as in the example of the broken line B in FIG. 3, when the driving torque does not become negative even in the region where the rotational speed of the driving motor 33 is low, the second derivative becomes zero, that is, the driving torque. The point where the value changes most steeply is determined as the inflection point. For example, the rotational speed of the drive motor 33 when the difference between the drive torque values at adjacent measurement points is the largest. Alternatively, the inflection point may be the midpoint of the range in which the drive torque changes sharply.

  When the rotational speed at which the drive torque of the drive motor 33 becomes the inflection point is determined in this way, the rotational speed is set as the rotational speed of the drive motor 33 during image formation. During image formation, the drive of the drive motor 33 is controlled with the rotational speed set in this way as a target. Accordingly, the relative speed with respect to the surface of each photoconductor 21 can be reduced as much as possible in consideration of the frictional force acting on the surface of the photoconductor 21.

  Note that this inflection point changes somewhat depending on durability. In addition to the rotational speed of the drive motor 33, the relative speed between the photosensitive member 21 and the intermediate transfer belt 11 includes the outer diameter of the belt driving roller, the outer diameter of the photosensitive member, the thickness of the belt, the belt driving roller and the back surface of the belt. It depends on the friction coefficient between Of these, for example, the thickness of the belt, the coefficient of friction, and the like change somewhat due to durability and environmental changes. Therefore, the determination of the inflection point and the setting process of the rotational speed of the drive motor 33 are not only performed when a new product is manufactured, but also when the environment changes (for example, when the normal environment changes to 30 ° C. or higher or 10 ° C. or lower). It is recommended to do this during durability such as every sheet printing, or during non-printing such as belt replacement.

  Next, the results of an experiment conducted by the inventor to confirm the effect of suppressing color misregistration by the adjustment method of this embodiment will be described. The color printer 1 of this embodiment is subjected to the above processing to set the rotation speed, and in that state, the registration marks of each color are printed on the paper, and the positional deviation for each color is measured. The result is shown in FIG. Here, the positional deviation from the theoretical printing position at a plurality of positions from the leading edge to the trailing edge of the paper is measured, and the line type is changed for each color. The color misregistration that is a problem in the present invention is a difference in the amount of misregistration between colors, and was about 0.07 mm at the maximum in this experiment.

  On the other hand, for comparison, a similar experiment was performed with the traveling speed of the intermediate transfer belt 11 being about 0.5% slower than the peripheral speed of each photoconductor. The result is shown in FIG. As can be seen from comparison with FIG. 4, the amount of shift between colors is large overall. In this comparative experiment, a maximum deviation of about 0.17 mm occurred. That is, according to the adjustment method of this embodiment, it was confirmed that the amount of color misregistration was considerably suppressed.

  As described above in detail, according to the color printer 1 of the present embodiment, the driving torque at that time is measured by changing the rotational speed, and the inflection point of the driving torque value is set as the target speed of the driving motor 33 at the time of image formation. Yes. Accordingly, it is possible to further reduce the speed difference between the intermediate transfer belt 11 and the photoreceptor as a whole. Since the deflection of the intermediate transfer belt 11 can be reduced, a large slip can be suppressed. As a result, even a transfer belt having a large coefficient of friction with the photosensitive member does not cause a large color shift.

In addition, this form is only a mere illustration and does not limit this invention at all. Therefore, the present invention can naturally be improved and modified in various ways without departing from the gist thereof.
For example, in this embodiment, a torque meter is provided to measure the driving torque, but the torque value can also be obtained by detecting the current value that flows through the driving motor 33. This is because a large current is required to generate a large torque value. In this case, the relationship between the current value of the drive motor 33 and the torque value may be measured in advance, and the current value corresponding to the state where the torque value becomes 0 is obtained.
Further, for example, in the present embodiment, the present invention is applied to a tandem color printer, but a so-called four-cycle image forming apparatus or monochrome image forming apparatus in which developing devices for each color are provided around one photosensitive member. It can also be applied to.

1 is a schematic configuration diagram illustrating a color printer according to an embodiment. FIG. 3 is an explanatory diagram showing an intermediate transfer belt and its drive system. 6 is a graph showing the relationship between the running speed of the intermediate transfer belt and the drive torque. It is a graph which shows the color shift state by an Example. It is a graph which shows the color shift state of a comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Color printer 10Y, 10M, 10C, 10K Image formation part 11 Intermediate transfer belt 21 Photoreceptor 33 Drive motor 34 Torque meter 35 Control part

Claims (6)

In an image forming apparatus, comprising: an image forming unit that forms a toner image on a rotationally driven photoconductor; and a transfer belt that is rotationally driven while being in contact with the photoconductor and onto which a toner image is transferred from the photoconductor ,
A drive unit for rotationally driving the transfer belt;
A torque measuring unit for measuring a driving torque value of the driving unit;
At the time of non-image formation, the rotational speed of the driving unit is changed, and the measured value of the torque measuring unit at each rotational speed is acquired. Based on the acquired result, the measured value is relative to the rotational speed of the driving unit. An inflection point determination unit that determines a rotation speed indicating an inflection point that changes most steeply, and at the time of image formation, the drive unit indicates the inflection point determined by the inflection point determination unit. An image forming apparatus, wherein the transfer belt is rotationally driven at a rotational speed. The image forming apparatus according to claim 1,
An image forming apparatus characterized in that the measurement of the torque measuring unit for determining an inflection point by the inflection point determining unit is performed while rotating the photosensitive member at the same rotational speed as in normal image formation. The image forming apparatus according to claim 1, wherein:
A belt cleaner that contacts the transfer belt and removes residual toner;
An image forming apparatus characterized in that the measurement of the torque measuring unit for determining an inflection point by the inflection point determining unit is performed while the belt cleaner is brought into contact with the same pressing force as in normal image formation. In the image forming apparatus according to any one of claims 1 to 3,
The inflection point determination unit
When the acquired drive torque value ranges from negative to positive, the rotational speed of the drive unit when the absolute value is the smallest is the inflection point,
When the acquired drive torque value does not vary from negative to positive, the rotation speed of the drive unit when the difference in drive torque between adjacent rotation speeds is the largest is the inflection point. Forming equipment. In the image forming apparatus according to any one of claims 1 to 3,
The transfer belt is an elastic belt having an elastic layer;
The inflection point determination unit uses the rotation speed of the drive unit when the absolute value of the acquired drive torque value is the smallest as an inflection point. In the image forming apparatus according to any one of claims 1 to 5,
An image forming apparatus comprising: the image forming unit for each color, and the photoconductor for each color is disposed along the transfer belt.

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