JPS6026839A - Suspending device for image carrying belt - Google Patents

Abstract

PURPOSE:To prevent endless guide members from being loosened upon releasing belt tension and facilitate a belt replacing work by providing an image carrying belt, a pair of the endless guide members, and two sets of tension applying means, in an image carrying belt suspending device of a picture image display device. CONSTITUTION:In a picture image display device, a pair of chains 14 wound around sprocket wheels 10 to 13 are rotated by driving of a motor M, and thereby a picture image carrying belt 9 mounted on a belt mounting member 18 hung between both chains 14 is rotated via supporting rollers 5-8. Then, the anticlockwise rotation of about 270 deg. of a lever 33 at time of replacement of belt 9 causes the whole of a tensioner side plate 29 to be turned around a joggle 31 by an eccentric cam 35, and thereby the roller 6 and the sprocket 11 are inwardly moved. Thereby, tension applied to the belt 9 is moderated, while the loosening of the chain 14 can be prevented via an arm 40 due to force of spring 43.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention converts image information converted into an electrical signal, which is calculated or read out from an electronic computer, image reading device, etc., or image information obtained using electrophotography, into a toner image. supported on an endless belt-like image carrier as
In addition to an image display device that reproduces and displays the image as it is, or an image forming device such as a copying machine that has the function of further transferring the image visualized on the belt to transfer paper, etc., especially a tension adding/releasing device for the image bearing belt. The present invention relates to an image carrying belt tensioning device comprising a tensioning device for the conveyance guide member of the belt.

Generally, image display devices and image forming devices having an endless image carrying belt as described above have a structure in which the image carrying belt is stretched around a plurality of rollers and rotated. In order to always rotate on a predetermined orbit while preventing waving or the like, a means is provided for applying an appropriate tension to the image carrying belt. In addition, a pair of endless guide members such as chains are provided on the tracks along both sides of the image carrying belt in the width direction to transmit the driving force for conveying the image carrying belt, or to position the image carrying belt at a predetermined position and device. This pair of endless guide members rotates on a trajectory in the same plane as the image carrying belt. Since it is necessary to move, tension is applied like the image carrying belt.

In general, it is preferable that the image carrying belt used in the above-mentioned image display device or image forming apparatus be detachably attached so that the image carrying belt can be easily replaced due to its service life. .

For this reason, the above-mentioned tension applying device has conventionally been provided with a means for releasing the tension on the belt in order to facilitate the replacement work of the image bearing belt.

This tension applying device applies tension to the pair of endless guide members located on both sides of the image carrying belt together with the image carrying belt, and when the tension on the image carrying belt is released by the tension releasing means, the pair of endless guide members are The tension in the endless guide member is also released. As a result, the endless guide member becomes loose, and when a chain or a timing belt is used, for example, a problem arises in that the chain or timing belt comes off from the chain sprocket or belt pulley, or the meshing position shifts.

These pair of endless guide members also have the role of aligning the main body of the apparatus and the image carrying belt in the image carrying belt transport pond as described above, and are generally provided at a predetermined position on the endless guide members. The operation of the main body of the device is controlled based on the timing when cams, protrusions, etc. located in the device operate, and when holes, etc. are read by sensors, etc. Therefore, if the endless guide member becomes disengaged from the sprocket, pulley, etc., or if the meshing position is simply shifted, the control of the main body of the device will be disrupted, resulting in an extremely inconvenient problem in that normal operation cannot be achieved.

In order to avoid this problem, it is necessary to correct these deviations every time the image carrying belt is replaced, which has the disadvantage that the image carrying belt replacement work becomes complicated.

In view of the above points, the present invention uses a simple method to prevent the pair of endless guide members from loosening that occurs when the tension of the image carrying belt is released, thereby solving the problems seen in the conventional example. The purpose is to solve all the problems.

The present invention will be explained based on embodiments shown in the drawings.

1 to 3 show the configuration of an image display device to which the present invention is applied, in which FIG. 1 is an external perspective view, FIG. 2 is a sectional perspective view, and FIG. 3 is a longitudinal sectional view.

1 is a vertical device exterior layer; 2 is a surface image viewing port formed with a large opening on the front panel surface of the exterior layer; 3 is an operation panel provided on the upper surface of the front panel with the lower part of the front panel protruding forward; 4 is window hole 2
Display window glass, 5, 6, 7, 8. are four endless belt-type photoreceptors (image-bearing belts) arranged parallel to each other in the left-right direction of the exterior layer and individually rotatably supported on bearings.
The suspended support roller 9 is an endless belt type photoreceptor which is suspended and supported by the four a-rollers.

In the endless belt type photoreceptor 9 (hereinafter simply referred to as belt), the base layer is, for example, a transparent sheet such as a polyester sheet, and a metal is deposited very thinly on the outer surface of the sheet to substantially maintain transparency. A photosensitive material layer (photoconductive layer) such as CdS is formed on the metal-deposited surface side, and the entire surface is flexible, with the photosensitive material layer side outside the belt track. In this example, among the four rollers, roller 5 is used as a driving roller, and the rotational force of a motor M (FIG. 3) is transmitted to the roller. At least one of the other three driven rollers, for example the a-ra 6, is applied a pressing force in a certain direction by springs 20 via bearings 19 at both ends (see Fig. 4), and is used as a belt tension roller. This action gives tension to the suspended belt.This tension mechanism will be explained in detail later.

! The above-mentioned hanging belt 9 is rotated by the motor M in the counterclockwise direction in FIG.
rotates counterclockwise in Fig. 3 without waving, sagging, or slipping, and with this rotation, the outer surface of the belt between rollers 5 and 8 (outer surface on the tension side of the belt) appears on the display image of the front plate of the exterior layer. Pass through the two viewing holes by moving from bottom to top.

Reference numeral 21 denotes a laser beam scanning type image exposure device which is fixedly disposed inside the orbit of the suspension belt 9, and includes a semiconductor laser oscillator 22, a polygon mirror scanner 23, an fθ lens 24, a reflecting mirror 25, and a device exterior. It is composed of a horizontally long transparent plate 26 made of glass, plastic, etc., which is arranged so as to make pressure contact with the inner surface of the tensioned belt part between the two lower inner belt a-time support rollers 7 and 8 with an appropriate force. .

This image exposure device 21 oscillates an intermittent laser beam L corresponding to time-series electric pixel signals outputted from an electronic computer, image reading device, etc. from a semiconductor laser oscillator 22 toward a polygon mirror scanner 23, and makes the laser beam L enter the scanner. The laser beam L is swung in the belt width direction and is incident on the inner surface of the belt portion between the rows 27 and 8 through the path of fθ lens 24→reflector 25→transparent plate 26, whereby the inner surface of the belt portion is oriented in the belt width direction. The laser beam is scanned. Then, this laser beam scanning is used as a main scanning, and the rotational movement of the belt 9 is used as a sub-scanning, so that the image of the belt 9 is exposed by the laser beam from the inner surface of the belt.

Reference numeral 27 denotes a toner developing device disposed on the outer surface side of the belt portion between the rollers 7 and 8.

The transparent plate 26 disposed on the inner side of the belt portion between the rollers 7 and 8, which serves as the laser beam scanning exposure section,
The lower surface of the plate 26 is slightly protruded downward from the common tangent line on the lower side of the plate 26, so as to be in suppressed contact with the inner surface of the belt, and the end surfaces 26. , 262 are chamfered and rounded so that the belt can come into contact with it smoothly. It functions to prevent the vertical vibration of the belt 9 caused by contact with the brush-like assembly of developer formed by the magnets I-, maintain the exposure position constant, and improve resolution. be.

When a display start command is input by setting necessary conditions such as calling up image information and specifying a display position by operating the operation panel 3, the rotation of the motor M causes the belt 9, which is a photoreceptor, to rotate at a predetermined speed. movement begins.

Further, the exposure device 21 starts scanning and exposing the belt 9 with designated image information using a laser beam on the inner side surface of the belt between the rollers 7 and 8. A toner developing device 27 is disposed on the outer surface of the belt in the exposure section, and toner acts on the outer surface of the belt at the same time as exposure, so that toner images corresponding to the exposed image are sequentially formed on the outer surface of the belt 9. . The toner images sequentially formed on the outer surface of the belt rotate from bottom to top to the display image viewing window hole 2 part of the exterior retractable front plate as the belt rotates, and once they move to the window hole range position, belt 9
rotation is stopped. As a result, an image is displayed on the window hole 2 (r), and the image is visible through the display window glass 4.Next, when the belt is rotated again by the belt re-rotation operation, the next display image is displayed at the window hole range position. The toner image on the outer surface of the belt after being displayed is cleaned and removed by a developing blank that reaches the toner developing unit 27 as the belt rotates. Immediately after this, a new toner image is formed by undergoing simultaneous exposure and development.

In this image display device, sprocket wheels 10, 11, 12.8 are provided as drive guide members for the image bearing belt near both ends of the support rollers 5, 6, 7.8, and coaxially with the rollers as shown in FIG. 13, and each of these sprocket wheels is provided with an endless chain 14.
I'm making a big deal of it. Further, one of the many ring plates of each chain 14 is connected to the ring plate facing the ring plate by the stay 16,
The image carrying belt 9 is attached to the stay 16 via a belt attachment member 18. Therefore, the driving force of the motor M is transmitted to the image carrying belt 9 via the chain 14 and the stay 16.
transmitted to. The image carrying belt is attached and detached between the stay 16 and the belt attachment member 18.

The image carrying belt tensioning device according to the present invention will be described below with reference to the accompanying drawings showing an embodiment of the tension applying and tension releasing mechanism for the image carrying belt according to the present invention applied to the apparatus having the above configuration. state

FIG. 4 shows the first part of the image bearing belt tensioning device according to the present invention.
FIG. 5, a partial perspective view of the embodiment, is an explanatory diagram of its operation.

First, the first tension applying and tension releasing device that constitutes a portion of the tension device will be described. tenyong roller 6,
and the sprocket wheel 11 are pivotally supported on a common shaft 28, which is supported by a bearing 19 on the tensioner side plate 2.
9 is rotatably supported. Tensioner side plate 29
A bearing 32 is attached to a dowel 31 fixed to the side plate 30 of the main body of the device.
Since the pi
The tensile force of 20 is transmitted to the shaft 28 using the dowel 31 as a fulcrum, and is applied to the tension roller 6 and sprocket wheel 1.
1 through the image carrying belt 9 and chain 14, respectively.
acts as a tension force. This tension is approximately several kg to several tens of kilograms
Requires relatively strong force.

Next, the lever 33 is rotatably supported by the main body side plate 30,
It protrudes forward from the hole 34 provided in the tensioner side plate 29 and has a curved shape as shown. An eccentric cam 35 is fixed to the lever 33, and the lever 33 and the eccentric cam 35 constitute a tension release device. This operation will be detailed later.

This image display device is provided with a second tension applying device in addition to the first tension applying device and the tension releasing device. i (that is, an arm 40 fixed to a shaft 39 rotatably supported on the main body side plate 30 via a bearing 38 is connected to a spring 4 hooked on a dowel 44 fixed to the main body side plate 30).
A sprocket wheel 42 is rotatably supported by a dowel 41 fixed near the tip of the arm, and the sprocket wheel 42 is
It meshes with 4. The spring 43 has an extremely weak tensile force compared to the spring 20, and has a tensile force of approximately 9 in several hundreds to 2 in several hundreds.

The first tension applying device, the tension releasing device, and the second tension applying device described above have devices having exactly the same configuration at opposing positions with the image bearing belt 9 interposed therebetween.

The operation will be explained below with reference to FIG.

When normal tension is applied to belt 9 and chain 14, lever 33 is in the position shown in FIG. 5(a). Next, to release the applied tension, lever 33
By rotating about 270° in the direction of arrow 36, the eccentric cam 35 pushes the bent portion 29a (see FIG. 4) provided on the tensioner side plate 29, and the tensioner side plate 2
The whole 9 moves in the direction of the arrow 37 using the dowel 31 as a fulcrum (see FIG. 5(b)). This movement causes the tension roller 6 and chain sprocket 11 to move inward toward the track surface of the belt 9, so that the force of the spring 20 acting on the belt 9 and chain 14 is lost. is transmitted to the chain sprocket 42 via the arm 40, and this force is transmitted to the chain sprocket 42 via the arm 40.
Since the chain 14 acts to prevent slack in the chain 14, the meshing state of the chain 14 is maintained as it is. However, since the force of the spring 43 does not act on the image carrying belt 9, the image carrying belt 9
There is no tension at all, making it easy to replace the belt 9.
State and 7 (l.

After replacing the belt in this state, the lever 33 is returned to the position shown in FIG. 5(a), thereby applying tension to the belt 9 and chain 14 again.

At this time, the force of pi 43 is applied to the chain 14, but as mentioned above, the force of the spring 43 is extremely weak compared to the spring 2o, so the image bearing belt 9 and the chain 14
The deviation of the orbital surface is completely negligible and does not affect the belt conveyance function of this image display device at all.

With the above configuration, even when the tension of the image carrying belt 9 is removed, tension is applied to the chain 14.
If the tension is released when replacing the belt, the chain 14 may come off from the chain sprocket 11, or the meshing state may change.Even after the belt is replaced, the meshing state of the chain remains the same as before the belt replacement, so this image shows the problem. The forming device can maintain a constant chain setting.

6 and 7 show a modification of the first embodiment according to the present invention, FIG. 6 is a perspective view of the vicinity of the dension roller, and FIG. 7 is an explanatory diagram of its operation.

In this modification, a timing belt 46 is used instead of the chain 14 in the above embodiment, and a timing pulley 45 is attached instead of the chain sprocket 11.

The shaft 28 is provided with a tension release device, similar to that shown in FIG. 4, which is pivotally supported by the first tension application device.
(Omitted in the figure) A leaf spring member 47 is fixed to the main body side plate 30, and a friction member 48 is adhered to the tip of the spring part.The friction member 48 is made of a highly wear-resistant and durable material such as Teflon felt. This member is constantly pressed against the timing belt 46 by the restoring force of the leaf spring member 47. Board pie・
The restoring force of the member is significantly weaker than the force of the spring 20 (see Figure 4), so the normal timing held 4
It has no effect on the rotation of 6.

FIG. 7(a) shows a state in which the timing belt 46 is applied with tension by the first tension applying device shown in FIG. The state shown in FIG. 7(b) is when the force from the device is applied. In the state shown in FIG. 7(b), only the restoring force of the plate spring member 47 is transmitted to the timing belt 46, This prevents changes in the engagement state with 45 etc.

With this configuration as well, effects equivalent to those of the embodiment described above can be obtained. In addition, in the first embodiment, the force of the second tension applying device is applied from the inside of the belt track surface, and in the above modification, it is applied from the outside, but in this way, in the present invention, the force is applied by the second tension applying device. The direction of the applied force may be either inside or outside the belt track surface and is not particularly limited.

Next, a second embodiment of the image carrying belt tensioning device according to the present invention will be described.

FIG. 8 is a partial perspective view of the above embodiment, and FIG. 9 is an explanatory diagram of its operation. Elements that are the same or similar to those in the first embodiment are given the same reference numerals as in the first embodiment. Further, since the structure and operation of the first tension applying device and the tension releasing device of the second embodiment are substantially the same as those of the first example, the explanation thereof will be omitted, and only the structure and operation related to the second tension applying device and the tension releasing device will be omitted. I will only explain the parts that will be used.

As shown in FIG. 8, an outer arm 40' and an inner arm 40'' are fixed to both ends of the shaft 39, respectively, and these are rotatably attached to the main body side plate 30 via a bearing 38. A sprocket wheel 42 is rotatably supported on a dowel 41 fixed near the tip of the inner arm 40'', and the sprocket wheel 42 meshes with the chain 14. A round hole 49 is opened in the outer arm 40 , and a sliding pin 50 is slidably engaged in the round hole 49 in the direction of arrow 51 and extends between the sliding pin 50 and the tensioner side plate 29 . The spring 52 is then locked.

When normal tension is applied to the belt 9 and chain 14 by the spring 20, the spring 52 slides in a slack state without any tension, as shown in FIGS. 8 and 9(a). It is passed between the pin 50 and the tensioner side plate 29. Further, a regulating dowel 55 is fixed to a round elongated hole 53 opened in the main body side plate 30 by a screw 54.

The above-described first tension applying device, tension releasing device, and second tension applying device have devices having exactly the same configuration at opposing positions with the image bearing belt 9 interposed therebetween.

Regarding its operation, as shown in FIG. 9, when normal tension is applied to the belt 9 and chain 14, the lever 33 is in the position shown in FIG. 9(a). At this time, since the spring 52 is in a slack state as shown in the figure, it does not act as an elastic body at all and does not exert any force on other parts.

The inner arm 40'' is stationary at a position where it abuts against the regulation dowel 55, and in this position, the sprocket wheel 42 is
Although it is engaged with the chain 14, tension is applied to the chain 14.

In order to achieve the above state, the position where the regulating dowel 55 is fixed with the screw 54 through the circular elongated hole 53 shown in Fig. 8 is adjusted in advance, and since this adjustment only needs to be done once, subsequent work will be done. It has no effect on sexuality.

To release the tension applied to the belt 9, the lever 33 is rotated approximately 270 degrees in the direction of the arrow 36, so that the eccentric cam 35 is released from the bent portion 29a provided on the tensioner side plate 29 (see FIG. 8). Press to move the entire tensioner side plate 29 in the direction of arrow 37 (Fig. 9(b)
reference). This movement causes the tension roller 6 and the chain sprocket 11 to move toward the inner side of the track surface of the belt 9, so that the force of the spring 20 acting on the belt 9 and the chain sprocket 14 is lost.

Further, as the tensioner side plate 29 moves, the spring 52 is pulled in the direction of the arrow 56, and this force is applied to the outside through the sliding pin 50 after sliding the sliding pin 50 to the end of the round elongated hole 49. outer arm 40' for pulling arm 40';
The shaft 39 and the inner arm 40'' rotate together in the direction of arrow 57 about the shaft 39, and this rotation causes the sprocket wheel 42 to push the chain 14.Therefore, the chain 14 is The tensile force of the spring 52 acts on the chain 14, and this force acts to prevent the chain 14 from slackening, so that the chain 14 remains engaged.However, the force of the spring 52 does not act on the image carrying belt 9. r (because of this, no tension is applied to the image bearing belt 9, and the belt 9 can be easily replaced).

After replacing the belt 9 in this state, by returning the lever 33 to the position shown in FIG. 9(a), the belt 9 and chain 14 are given tension by the spring 20.

At this time, the spring 52 becomes slack again, so the chain 14 receives no force from the sprocket wheel 42
(stomach.

With the above configuration, tension is applied to the chain 14 even when the tension of the image bearing belt 9 is removed, so that when the tension is released when replacing the belt, the chain 14 is attached to the sprocket wheels 10, 11, 12° 13. If the belt has come off or the engagement status has changed, even after replacing the belt,
The meshing state of the chain 14 has changed from before the belt was replaced7(
Therefore, this image display device can maintain a constant setting state.

10 and 11 show a modification of the second embodiment of the present invention, with FIG. 10 being a perspective view of the tension roller 6 and its vicinity, and FIG. 11 being an explanatory diagram of its operation.

A lever 33' rotatably supported by the main body side plate 30 has a curved shape as shown in FIG. 10, and an eccentric cam 35' is fixed thereto. The eccentric cam 35' is arranged so that the lever 33' is the first
When in the position shown in FIG. 1(a), the bent portion 40a provided on the outer arm 40' is pushed.

Also, unlike the second embodiment, the spring 20 is hooked onto the outer arm 40'. For this reason, the outer arm 40
1, the second tensioning device consisting of the shaft 38 and the inner arm 40'' is fixed in the position shown in FIG. is transmitted to the chain 14 and belt 9 as tension.

In this case, the sprocket wheel 42 is in mesh with the chain 14, but is not pushing on the chain 14, and the second tensioning device does not tension the chain 14.

When lever 33' is rotated in the direction of arrow 58 in the state shown in FIG. 11(a), the tensile force of pi 20 causes the entire second tension applying device to rotate in the direction of arrow 59 about shaft 39 as a fulcrum. At this time, since the tensile force of the spring 20 is weakened, the tensioner side plate 29 rotates about the headerbob 31 in the direction of arrow 37 due to the weight of the roller 6, sprocket 11, etc. as a fulcrum.

The above two types of rotation are caused by the weight of the roller 6 etc.
The sprocket wheel 42 stops at a position where the moment about the shaft 39 and the moment about the shaft 39 created by the reaction of the force of the sprocket wheel 42 pushing on the chain 14 are balanced.

In this state, that is, the state shown in FIG. 11(b), the chain 14 is being pushed by the sprocket wheel 42, so tension is acting on the chain 14.
No. 4 does not sag and the meshing state is maintained.

However, since tension is not applied to the belt 9, the belt 9 is easily replaced in the y and z states.

After replacing the belt 9 in this state, by returning the lever 33' to the position shown in FIG. 9(a), the tensile force of the spring 20 acts on the belt 9 and the chain 14 as tension.

The above configuration also provides the same effects as the second embodiment. rz Oh, in the above embodiment, the spring 20 that applies force to the first tension applying device and the spring 52 that applies force to the second tension applying device are provided, and each act independently. - 20 is applying force to both the first and second tensioning devices.

As described above, in the present invention, the first and second tension applying devices may be separately provided with a piezoelectric device for applying tension, or both may be used in common.

In addition, in the above four examples, the chain 14 is used as an endless belt suspended on tracks provided on both sides of the belt 9, but a device that drives the belt 9 by other methods such as a timing belt is also described. Similar effects can be obtained by using the present invention.

Furthermore, in the above four examples, a coil spring is used as a means for applying tension, but similar applications can be considered using an elastic body such as a plate spring member or rubber, and in particular, the means for applying tension is limited. 7 (It's obvious.

Furthermore, in the above four examples, the second tension applying device was provided at only one location, but by providing the second tension applying device at multiple locations, the second tension applying device is applied to each part of the chain 14 when tension is released. The structure may be such that the tension acts almost equally.

Further, in the above four examples, the present invention is applied to an image display device, but the present invention can also be applied to an image forming device such as a copying machine using an endless image bearing belt. It is clear that the effects described above can be obtained.

The present invention is configured as described above, and for example, in the tensioning means for the image carrying belt, when the tension of the first tension applying means for the image carrying belt is released by the tension releasing means, the tension of the image carrying belt is Since tension is applied to the endless guide member for conveyance by the second tension applying means, the tension release action of the endless guide member accompanying the release action is prevented, and furthermore, fluctuations in the operating position of the guide member relative to the device are prevented. Problems can be avoided and the image bearing belt can be properly and easily maintained and replaced, which is extremely effective.

[Brief explanation of the drawing]

1 is an external perspective view of an image display device that is an applied example of the present invention, FIG. 2 is a cross-sectional perspective view of the image display device shown in FIG. 1, and FIG. 3 is an image display device shown in FIG. 1. FIG. 4 is a longitudinal sectional view of the display device, and FIG. 5 is a perspective view of the vicinity of the tension roller of the first embodiment of the image bearing belt tensioning device according to the present invention.
FIG. 4 is an explanatory diagram of the operation of the device, in which (a) shows when tension is applied (
b) shows the state when the tension is released, FIG. 6 is a perspective view of the vicinity of the tension roller of a modification of the device shown in FIG. 4, and FIG. 7 is an explanatory diagram of the operation of the modification. When added, (b)
8 shows a state when the tension is released, FIG. 8 is a perspective view of the vicinity of the tension roller of the second embodiment of the image carrying belt tensioning device according to the present invention, and FIG. 9 is an explanation of the operation of the device shown in FIG. 8. In the figure,
(a) shows the state when tension is applied, (b) shows the state when tension is released, FIG. 10 is a perspective view of the vicinity of the tension roller of a modification of the device shown in FIG. 8, and FIG. 11 shows the operation of the modification. In the illustration,
(a) shows the state when tension is applied, and (b) shows the state when tension is released. 5... Drive roller 6... Tension roller 78.
...Roller 9...Image carrying belt 10.11,12.
13... Sprocket wheel 14... Chain
20°° Spring 28...Shaft 29...Tensioner side plate 30...
Device main body side plate 31... Dowel 33. 33'... Lever 35, 35'... Eccentric cam 39... Shaft 40.
...Arm 40'...Outer arm 40"...Inner arm 42
...Sprocket wheel 43...Spring 45...Timing pulley 46...
・Timing belt 47...Plate spring member 48...Friction member 49...Actual elongated hole 50...Sliding pin 52・
・・・Pie・53・・Actual oblong hole 54・screw 55・dowel 3rd figure 4th figure (a) (b) 7511th 9th! l Figure 10 Figure 11 (b)

Claims (1)

[Claims] An image carrying belt that is stretched and rotated between a plurality of parallel rollers, and a pair of supports that rotate on support bodies that are provided concentrically on both sides of the parallel rollers to guide the image carrying belt. an endless guide member; a first tension applying means for simultaneously applying and releasing tension to both the image bearing belt and the endless guide member; A tensioning device for an image bearing belt having a second tension applying means for applying a different tension every month.