JP2002182537A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device where degradation of the picture quality and erroneous detection of a color deviation detection pattern is reduced by decreasing flaws generated by the difference of the speeds of the photoreceptor drum and the carrier belt. SOLUTION: The image forming device is typically provided with an image carrier which carries an image, a transfer means which transfers the toner image on the image carrier to an endless belt or to the recording material carried on the endless belt, a driving means for driving the image carrier or the endless belt, and a coupling means that transmits driving power from the driving means independently to the image carrier or the endless belt. When the driving means is activated, driving is conducted at a speed lower than image forming speed until the coupling means having the longest fitting time among various coupling means fits in the state that the image carrier is in contact with the above endless belt.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a color printer,
The present invention relates to an image forming apparatus having a plurality of image carriers, such as a color copier, and more particularly, to controlling the rotational drive of an image carrier.

[0002]

2. Description of the Related Art Conventionally, an electrophotographic image forming apparatus has a plurality of photosensitive drums as image carriers for speeding up, and a recording material carried on a transport belt as an endless belt. Various methods for sequentially transferring toner images of different colors have been proposed. Problems with such an apparatus having a plurality of photosensitive drums include uneven movement of the plurality of photosensitive drums and the conveyor belt due to mechanical accuracy and the like, and the relationship between the outer peripheral surface of the photosensitive drum and the amount of movement of the conveyor belt at each transfer position. And the like occur separately for each color, do not match when images are superimposed, and cause color shift.

FIGS. 11A to 11D show examples of color shifts as color shifts 101 a to 101 d with respect to the original image position 100. FIGS. 11B and 11C show the case where there is a color shift in the main scanning direction, but two lines are drawn apart in the transport direction for the sake of explanation.

FIG. 11A shows an inclination shift 101a of the main scanning line, which occurs when there is an inclination between the optical unit and the photosensitive drum. This is, for example, the position of the optical unit and the photosensitive drum,
By adjusting the position of the lens, it can be corrected in the direction of the arrow. FIG. 11B shows a color shift 101b due to variations in the width of the main scanning line, which is caused by a difference in the distance between the optical unit and the photosensitive drum, and is particularly likely to occur when the optical unit is a laser scanner. This can be corrected in the direction of the arrow in the figure by, for example, finely adjusting the image frequency (if the scanning line width is long, increasing the frequency) and changing the length of the scanning line. FIG. 11C shows a shift 101c due to a writing start position error in the main scanning direction. For example, if the optical unit is a laser scanner, it can be corrected in the direction of the arrow in the figure by adjusting the writing start timing from the beam detection position. FIG. 11D shows a writing position error in the sheet conveying direction. This can be corrected in the direction of the arrow, for example, by adjusting the writing start timing of each color from the sheet leading edge detection.

In order to correct these color misregistrations, a color misregistration detection pattern is formed for each color on a transport belt 102, which is an endless belt, as shown in FIG. The detection is performed by a pair of optical sensors 103 provided, and the above-described various adjustments are performed according to the detected shift amount. In the figure, patterns 104a-d, 105a-
d, patterns 106a-d and 107a-d for detecting the amount of color shift in the main scanning direction orthogonal to the sheet conveying direction are formed, and black (K), yellow (Y), magenta (M), It corresponds to cyan (C).

In the above construction, the photosensitive drum and the transport belt drive roller are connected to the drive shaft of the main body via a coupling so that the user can replace the photosensitive drum (for example, a cartridge type) and the transport belt. Have been. FIG.
FIG. 6 is a diagram for explaining a motor and a photosensitive drum or a conveyance belt surface speed at the time of starting a motor as a driving unit. In the figure, when the coupling is engaged, the surface speed of the photosensitive drum or the conveying belt is proportional to the motor speed. Further, there is no separating mechanism between the photosensitive drum and the transport belt, and the surface is always in contact.

Here, conventionally, a device is devised so that a speed difference between the surface of the photosensitive drum and the surface of the conveyor belt does not occur when the motor is started. For example, when the photosensitive drum and the conveyance belt are driven by different motors, the starting curves of the respective motors are set so that the surface speeds of the photosensitive drum and the conveyance belt at the time of starting are the same. In some cases, the photosensitive drum and the conveyor belt are driven by the same motor, so that the surface speed becomes the same regardless of the starting curve of the motor.

[0008]

However, in the above configuration, as shown in FIG. 13, when the coupling is not engaged, the motor rotates until the coupling is fitted at the time of starting the motor. Despite this, the photosensitive drum or the conveyor belt does not start rotating, and the surface speed is zero. When the coupling is fitted, the photosensitive drum or the conveyance belt starts rotating at a speed corresponding to the motor rotation speed at that time.

[0009] The time (rotation angle, moving distance) from the start of the motor to the engagement of the coupling is determined by the drive shaft of the photosensitive drum and the conveyor belt drive roller at the time of start of the motor and the coupling of the drive shaft of the main body. Depends on the positional relationship. For example, the longest time (rotation angle and moving distance) until the coupling is fitted is 1/4 rotation of the photosensitive drum or the drive roller, the motor start time is 2 rotations of the motor, and the reduction ratio of the drive train is 1/10. In this configuration, the motor is started while the coupling of the drive roller is fitted, and the coupling of the photosensitive drum is fitted in the longest time. Then, the drive roller rotates 1/5 rotation (= 2 rotation / 10) before the motor reaches the steady speed, and furthermore,
After 1/20 rotation (= 1 / 4-1 / 5 rotation), the coupling of the photosensitive drum is fitted. That is, while the surface speed of the photosensitive drum remains zero, the surface speed of the transport belt reaches a steady speed, and the photosensitive belt surface is rubbed by the transport belt while the drive roller makes a quarter turn.

FIG. 14 is a diagram illustrating scratches on the surface of the photosensitive drum or the conveyor belt. Since the photosensitive drum 108 and the surface of the conveyor belt are in contact with each other, if the surface speed difference is large, the photosensitive drum 108 rubs against each other, causing scratches. For example, as shown in the figure, when only the transport belt rotates with the photosensitive drum 108 stopped, the stopped photosensitive drum 108 has a belt-like shape according to the nip width between the photosensitive drum and the transport belt and the length of the photosensitive drum. Scratch 109 occurs. Similarly, when the conveyance belt stops and only the photosensitive drum 108 rotates, the conveyance belt is similarly damaged.

When such scratches occur, the formation and development of the latent image on the photosensitive drum are not performed normally, and the image quality is degraded. Fig. 15 shows the case where there is a scratch on the conveyor belt.
7 shows detection signals of the color misregistration detection patterns 104 to 107. When the flaw becomes large or deep, a signal equivalent to the color misregistration detection pattern detection signal is detected in the flaw portion in addition to the original color misregistration detection pattern. In particular, when the detection means of the color misregistration detection pattern is configured using a regular reflection optical system,
Susceptible to scratches.

Accordingly, the present invention is to provide an image forming apparatus in which the occurrence of scratches caused by the speed difference between the photosensitive drum and the conveyor belt is reduced, thereby reducing image quality deterioration and erroneous detection of color misregistration detection patterns. The purpose is.

[0013]

In order to solve the above-mentioned problems, a typical configuration of an image forming apparatus according to the present invention comprises an image carrier for carrying an image and an endless toner image on the image carrier. Transfer means for transferring to a recording material carried on a belt or an endless belt; drive means for driving the image carrier or the endless belt; and drive transmission from the drive means independently of the image carrier or the endless belt. In the image forming apparatus having the coupling means, when the driving of the driving means is started, in a state where the image carrier is in contact with the endless belt, the coupling time is the longest of the respective coupling means. Until the coupling means is fitted, the driving means is driven at a lower speed than the image forming speed.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment A first embodiment of an image forming apparatus according to the present invention will be described with reference to the drawings. FIG. 1 is a schematic configuration diagram illustrating the entire configuration of the image forming apparatus according to the present embodiment, FIG. 2 is a diagram illustrating a drive transmission system of an image carrier and an endless belt, and FIG. 3 is a diagram illustrating each drive transmission system. FIG. 5 is an explanatory diagram of the coupling means, FIG. 6 is a diagram for explaining the rotation control of the motor, and FIG. 7 is a diagram for explaining the surface speed of the image carrier or the endless belt when the motor is started.

As shown in FIG. 1, an image forming apparatus according to the present embodiment includes a four-color photosensitive drum 1 as a plurality of image carriers.
a to d. The photosensitive drums 1a to 1d correspond to yellow (Y), magenta (M), cyan (C), and black (K), respectively, and carry toner images of respective colors obtained by developing the formed electrostatic latent images. A laser scanner 2 is arranged near the photosensitive drum 1 and forms an electrostatic latent image by scanning the photosensitive drum 1 with a laser beam.

All photosensitive drums 1 are in contact with a conveyor belt 3 which is an endless belt. The conveyor belt 3 sequentially conveys the sheet S to the photosensitive drums 1a to 1d of each color.
It is stretched by a driving roller 4 and a driven roller 5.
The driving roller 4 is connected to a motor 7 by a reduction gear 8 or the like (see FIG. 2), and transmits driving force. A color misregistration detection pattern (not shown) is formed on the conveyor belt 3 and a pair of optical sensors 6a and 6b for detecting the color misregistration detection pattern.
Are provided on both sides of the conveyor belt 3.

When image information is transmitted from a host computer (not shown), a sheet S is supplied from a feeding means (not shown), and is sequentially conveyed to the photosensitive drums 1 of each color by a conveying belt 3. Image signals of the respective colors are sent to the laser scanners 2a to 2d at the same time as the sheet conveyance by the conveyor belt 3, and electrostatic latent images are formed on the photosensitive drums 1a to 1d, which are developed by a developing unit (not shown). As a result, toner images of the respective colors are formed, and are superimposedly transferred onto the sheet S on the transport belt 3 by a transfer unit (not shown).
The sheet S to which the toner image has been transferred is fixed by applying heat and pressure by a fixing device (not shown), and is discharged outside the apparatus, thereby completing the image formation.

As shown in FIG. 2, a photosensitive drum 1a of each color
-D and the drive roller 4 of the transport belt 3 are connected to independent motors 7a-e via reduction gears 8a-e, respectively. However, the configuration (reduction ratio and the like) of each reduction gear 8 is the same. The surfaces of the photosensitive drum 1 and the conveyor belt 3 are always in contact, and each surface rotates in the same direction at substantially the same speed.

As shown in FIG. 3, the drive roller 4 of the photosensitive drum 1 and the transport belt 3 is driven by a motor 7 which is a drive unit. The speed from the motor shaft 7a to the drive shaft 9 is reduced by the reduction gear 8, and the drive torque is increased. The photosensitive drum 1 is of a cartridge type, and a coupling portion 23 is provided so that the user can easily replace the photosensitive drum.
Is connected to the drive shaft 9 via the. Also the conveyor belt 3
Is shorter in life than the main body of the apparatus, and is connected to the drive shaft 9 via the coupling portion 10 like the photosensitive drum 1 in order to allow the user to replace the apparatus. The coupling parts 10 and 23 are detached by being pulled out in the axial direction of the photosensitive drum 1 and the drive roller 4.

FIG. 4 shows the coupling of the photosensitive drum 1. The coupling portion 23, which is the first coupling means, includes a pair of coupling concave portions 23a and convex portions 23b, each of which forms a triangular twist structure. When the motor 7 starts rotating, the coupling concave portion 23a on the main body side starts rotating, and the fitting gradually deepens according to the triangular torsion structure with the coupling convex portion 23b. Is fitted in the concave portion 23a, and the drive is transmitted to the photosensitive drum 1 via the coupling portion 23.

FIG. 5 shows the coupling of the driving roller 4. The coupling unit 10, which is a second coupling means, includes a pair of couplings 10a and 10b.
Holes 10c are formed in 10a at intervals of 90 degrees. Correspondingly, projections 10d are formed on the coupling 10b at intervals of 90 degrees. When the motor 7 starts rotating, the coupling 10b on the main body side starts rotating, and the protrusion of the coupling 10b.
When 10d reaches the position of the hole 10c of the coupling 10a, the projection fits into the hole, and the drive is transmitted to the drive roller 4 via the coupling portion 10.

FIG. 6 is a diagram for explaining the rotation control of the motor 7. In this embodiment, the motor 7 is a stepping motor, and its start and stop is instructed by the CPU 11. A slow-up / down data holding unit 12 and a phase signal generation unit 13 are connected to the CPU. The pulse cycle of the motor 7 is held in the slow-up / down data holding unit 12, and the phase signal generation unit 13 generates a phase signal according to the pulse cycle from the slow-up / down data holding unit 12. The driver 14 drives the windings of the motor according to the phase signal from the phase signal generation unit, and the motor 7 rotates.

Next, rotational drive control in the image forming apparatus having the above configuration will be described with reference to FIG. As described above, when the motor is started, the concave portion 23a of the coupling portion 23 is formed.
And the projection 23b or the hole 10c and the projection 10d are fitted together.
Although the motor 7 is rotating, the photosensitive drum 1 or the conveyor belt 3 does not start rotating, and the surface speed is zero. The concave portion 23a and the convex portion 23b of the coupling portion 23 or the hole 10
When c and the projection 10d match, the photosensitive drum 1 or the conveyor belt 3 starts rotating at a speed corresponding to the motor rotation speed at that time.

The time (rotation angle and moving distance) from the start of the motor start until the concave portion 23a and the convex portion 23b or the hole 10c and the projection 10d of the coupling portion 23 coincide with each other is determined by the concave portion 23a and the convex portion when the motor is started. 23b or it depends on the positional relationship between the hole 10c and the projection 10d. When the reduction ratio of the reduction gear 8 is 1/10, since the concave portion 23a and the convex portion 23b coincide with each other when rotated at a maximum of 120 degrees, the maximum time until the coincidence is 3.3 rotations of the motor (= 120 degrees).
/ 360 degrees * 10). On the other hand, the hole 10c and the projection 10d have a maximum of 90
Since the rotation is the same, the maximum time for the rotation is 2.5 rotations of the motor (= 90 degrees / 360 degrees * 10).

In this embodiment, in order to obtain a stable fixing property, the image forming speed is 1st speed, 1/2 speed for thick paper such as coated paper, and paper used by transmitting light with OHP or the like. For 1/4 speed. As shown in FIG. 7, the motor rotates 3.3 times (200 steps / motor 1) after the start of the motor until the coupling of the photosensitive drum 1 having a long fitting time is fitted.
During the period (3.3 * 660 steps), the pulse period is rotated at a constant speed lower than 1/4 speed, and the motor 7 is not accelerated. Then, slow-up is performed to accelerate to a steady speed.

The above control is similarly performed for the independent motors 7a to 7e in common. Accordingly, in the photosensitive drums 1a to 1d and the transport belt 3 of each color shown in FIG.
b or the hole 10c and the protrusion 10d coincide with each other or about 120
However, even if the distance is about 90 degrees or so, the difference in surface speed at the time of starting the motor can be kept small, and scratches generated on these surfaces can be reduced.

[Second Embodiment] A second embodiment of the image forming apparatus according to the present invention will be described. FIG. 8 is a diagram illustrating a drive transmission system for an image carrier and an endless belt according to the present embodiment, and FIG. 9 is a diagram illustrating drive control of a motor. Are denoted by the same reference numerals and description thereof is omitted.

In the above-described first embodiment, independent motors 7a to 7e are provided for the photosensitive drums 1a to 1d of the respective colors and the drive roller 4 of the transport belt 3, respectively. This is an example in which means are provided.

As shown in FIG. 8, the photosensitive drum 1a of each color
.. D and the drive roller 4 of the transport belt 3 are provided with a motor 15 as a common drive means, and are connected to each other by a reduction gear 16. The surfaces of the photosensitive drums 1a to 1d and the conveyor belt 3 are always in contact, and each surface rotates in the same direction at substantially the same speed.

As shown in FIG. 9, the motor 15 is a DC motor (for example, a brushless motor of an outer rotor) in the present embodiment, and its start / stop is instructed by the CPU 17. Motor rotation state detection unit 18 (for example, MR
Magnetic encoder with blocking)
The PI controller 19 performs arithmetic processing so as to keep the number of rotations of the motor 15 constant, and adjusts a pulse width (motor supply power) supplied to the driver 21. Driver 21 has a pulse width controller
Motor 15 according to the pulse width modulation signal (PWM) from
The power supply to the system. Further, the winding for supplying electric power is switched according to the signal from the phase position detection unit 22 (brushless operation), and the motor 15 rotates.

After the start of the motor, the target speed of the PI control unit 19 is rotated at a constant speed lower than 1/4 speed while the motor 15 rotates 3.3 times as in the first embodiment.
Do not accelerate. After that, the target speed of the PI control unit is changed to the steady speed and acceleration is performed.

By using a common drive means as described above, the structure of the apparatus can be simplified and the apparatus can be downsized, and the surface speed can be easily made the same regardless of the starting curve of the motor.

[Third Embodiment] A third embodiment of the image forming apparatus according to the present invention will be described. FIG. 10 is a diagram for explaining the flow of processing at the time of starting the motor. Parts that are the same as those described in the first or second embodiment are denoted by the same reference numerals, and description thereof is omitted.

As described above, the positional relationship between the concave portion 23a and the convex portion 23b of the coupling portion 23, or the positional relationship between the hole 10c and the projection 10d becomes arbitrary after the photosensitive drum 1 or the conveyor belt 3 is inserted and removed. However, once the motor is driven, the concave portion 23a and the convex portion 23b of the coupling portion 23 or the hole 10c and the projection 10d
Are matched, the matching state is maintained until the photosensitive drum 1 or 3 is removed and inserted again. Here, when the photosensitive drum 1 or the conveyor belt 3 is removed or inserted, the power is turned off / on, or the cover of the apparatus body is opened,
And it is closed.

Therefore, in this embodiment, the slow drive control of the drive means described in the first or second embodiment is not performed every time. Then, as shown in FIG. 10, when there is a motor start start instruction (S1), it is first determined whether or not the power has been turned on or the cover has been closed (S2). Then, the slow drive control (S3) is performed only when the power is turned on, and otherwise, the normal startup (S4) similar to the conventional one is performed, and the operation shifts to the steady speed operation (S5).

With this configuration, it is possible to perform slow drive control only when the motor is started, and it is possible to reduce scratches on the image carrier and the endless belt and to reduce the start speed as much as possible. Can be minimized.

[Other Embodiments] In each of the above embodiments, the description has been made of the configuration having four photosensitive drums.
It may be configured to have one photosensitive drum. Further, the configuration has been described in which all the photosensitive drums 1 of four colors are always in contact with the conveyor belt 3, but the photosensitive drums 1 of color (Y, M, C)
a to c are provided with a mechanism for separating from and moving from the conveyor belt 3, and black (K)
Only the photosensitive drum 1d may always be in contact with the conveyor belt. In addition, the four photosensitive drums have the same shape and the conveyance belt has a different coupling shape. However, the conveyance belt has no coupling means, and of the four photosensitive drums, K and YMC have different coupling shapes. A configuration may be used.

Although the case has been described in which the drive transmission system is a stepping motor independent of each color and a DC motor common to each color, the configuration of the combination of the reduction gear 8 and the motor 7 can be arbitrary. Further, the slow drive control is not necessarily required to be a constant speed as long as the speed is low until the coupling units 10 and 23 are fitted. In addition, the slow drive control is described to be performed only after the power is turned on or after the cover is closed. It may be configured to perform drive control.

In each of the above embodiments, the transfer belt is used as an example of the endless belt, and the toner image on the photosensitive drum is transferred to the recording material carried on the transfer belt. However, using an intermediate transfer belt as an endless belt, an image forming apparatus configured to temporarily transfer the toner image on the photosensitive drum onto the intermediate transfer belt in a superimposed manner and collectively transfer the toner image separately to a separately fed recording material Even
The present invention can be applied.

[0040]

As described above, in the image forming apparatus according to the present invention, the difference in surface speed between the image carrier and the endless belt can be reduced by performing the slow drive control when the motor is started. It is possible to provide an image forming apparatus in which scratches caused by a speed difference can be reduced, and deterioration of image quality and erroneous detection of a color misregistration detection pattern are reduced.

By performing the slow drive control only after the power is turned on or after the cover is closed, the scratches on the image carrier and the endless belt can be reduced, and the reduction in the starting speed can be minimized.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram illustrating an overall configuration of an image forming apparatus according to a first embodiment.

FIG. 2 is a diagram illustrating a drive transmission system for an image carrier and an endless belt.

FIG. 3 is a diagram illustrating each drive transmission system.

FIG. 4 is an explanatory view of a coupling means.

FIG. 5 is an explanatory view of a coupling means.

FIG. 6 is a diagram illustrating rotation control of a motor.

FIG. 7 is a diagram illustrating a surface speed of an image carrier or an endless belt when a motor is started.

FIG. 8 is a diagram illustrating a drive transmission system for an image carrier and an endless belt according to a second embodiment.

FIG. 9 is a diagram illustrating drive control of a motor.

FIG. 10 is a diagram illustrating a flow of a process at the time of starting a motor.

FIG. 11 is a diagram illustrating an example of color misregistration.

FIG. 12 is a diagram illustrating a pattern for detecting color misregistration.

FIG. 13 is a diagram illustrating a motor and a photosensitive drum or a conveying belt surface speed according to a conventional example.

FIG. 14 is a diagram illustrating scratches on the surface of the photosensitive drum or the conveyance belt.

FIG. 15 is a diagram illustrating a color misregistration detection pattern detection signal when a flaw is present on the transport belt.

[Explanation of symbols]

 S ... sheet 1 ... photosensitive drum 2 ... laser scanner 3 ... transport belt 4 ... drive roller 5 ... driven roller 6 ... optical sensor 7 ... motor 8 ... reduction gear 9 ... drive shaft 10 ... coupling part 10c ... hole 10d ... projection 11 … CPU 12… Slow up / down data holding unit 13… Phase signal generation unit 14… Driver 15… Motor 16… Reduction gear 17… CPU 18… Rotation state detection unit 19… PI control unit 20… Pulse width control unit 21… Driver 22 … Phase position detection part 23… Coupling part

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification FI FI Theme Court ゛ (Reference) F16D 1/02 L F Term (Reference) 2H027 DA26 DA35 EB04 ED01 ED24 EE03 EE04 EF01 2H030 AA01 AB02 AD17 BB02 BB23 BB53 BB56 2H032 AA15 BA09 BA18 CA02 CA13 2H071 CA01 CA02 CA05 DA09 DA15 DA23 DA26 DA27 DA31 EA18

Claims (12)

[Claims] An image bearing member for carrying an image; transfer means for transferring a toner image on the image bearing member to an endless belt or a recording material carried on the endless belt; and a driving force applied to the image bearing member. An image forming apparatus comprising: a first coupling unit that transmits a driving force; a second coupling unit that transmits a driving force to an endless belt; and a driving unit that applies a driving force to the first and second coupling units. In the apparatus, the driving unit is driven at a lower speed than the image forming speed from the start of driving until the coupling unit having the longest coupling time among the first and second coupling units is coupled. An image forming apparatus comprising: 2. The image forming apparatus according to claim 1, wherein the plurality of image carriers and the first coupling unit transmit driving force to the plurality of image carriers. 3. The coupling device according to claim 1, wherein the coupling unit has a plurality of fitting positions, and the fitting time is a moving time between the plurality of fitting positions. Image forming apparatus. 4. The image forming apparatus according to claim 1, wherein the image forming speed has a plurality of speeds, and the low speed is lower than the lowest image forming speed among the plurality of speeds. 3. The image forming apparatus according to 2. 5. The image forming apparatus according to claim 1, wherein the driving unit is provided independently of the image carrier or the endless belt. 6. The image forming apparatus according to claim 1, wherein the driving unit is provided commonly to the image carrier or the endless belt.
And the image forming apparatus according to 2. 7. The image forming apparatus according to claim 1, wherein the driving unit is a stepping motor. 8. The image forming apparatus according to claim 1, wherein the driving unit is a DC brushless motor. 9. The image forming apparatus according to claim 2, wherein said first coupling means is provided independently for each of said plurality of image carriers. 10. The image forming apparatus according to claim 2, wherein the first coupling unit is provided for each of the plurality of image carriers. 11. The method according to claim 1, wherein the operation of driving the driving unit at a low speed is performed only at the start of driving of the driving unit after power is turned on or after a cover of the apparatus main body is closed. An image forming apparatus according to claim 1. 12. The apparatus according to claim 1, wherein the operation of driving the driving unit at a low speed is performed only when the driving of the driving unit is started after the image carrier or the endless belt is attached or detached. An image forming apparatus according to claim 1.