1.  Field of the Invention
The present invention relates to an image forming apparatus such as electro-photographing apparatus, printer, etc.  and in particular to an image forming apparatus equipped with a photo-conductor and developing rollers which are brought in contact with each other only when developing images for forming developing images on the photosensitive belt. 
2.  Description of the Related Art
In image forming apparatus such as color copying machines, color printers, etc.  to obtain color images according to electro-photographing system, an apparatus has been developed for obtaining a full color image using 4 color toners of yellow (Y), magenta (M), cyan (C), black (K).  In this apparatus, four developing devices filled with respective color toners are brought in contact with or separating from a photosensitive belt or an intermediate transferring body according to colors of latent images to be formed on the photosensitive belt and after developing and superposing toner images in respective colors on the photosensitive belt or the intermediate transferring body, the images are collectively transferred on a recording paper. 
In such the image forming apparatus, when bringing the photosensitive belt and plural developing devices are brought in contact with or separated from the photosensitive belt or separated therefrom, while the photosensitive belt is kept stationary, one of plural developing devices is moved or while the developing devices is kept stationary, the photosensitive belt is moved selectively to a position opposing to a prescribed developing device of the photosensitive belt, and the photosensitive belt and the developing rollers are thus contacted or separated. 
However, when toner images in desired colors are formed on the photosensitive belt in order by bringing the photosensitive belt in contact with developing rollers or separating them, even when either the photosensitive belt or the developing devices are moved, the tensile force of the photosensitive belt differs when the photosensitive belt is in contact with the developing rollers or when they are separated, and the running speed of the photosensitive belt may fluctuate. 
Therefore, as an image forming apparatus to move the developing devices to bring in contact with the photosensitive belt or separating from it, the technology to prevent fluctuation in the running speed of the photosensitive belt by keeping its tensile force constant by providing a roller to contact to the photosensitive belt or separate therefrom in addition to supporting rollers for stretching the photosensitive belt has been disclosed in Japanese Laid Open Patent Publication No.  Hei 11-167277.  Further, as an apparatus for moving the photosensitive belt to contact or separate to/from the developing devices, the technology to prevent fluctuation in the running speed of the photosensitive belt by moving plural back-up rollers corresponding to respective developing devices while keeping the running route of the photosensitive belt constant has been disclosed in Japanese Laid Open Patent Publication No.  Hei 9-80860. 
However, the existing technologies mentioned above maintains the tensile force of the entire photosensitive belt constant or maintains the running route of the entire photosensitive belt constant but do not adjusts the pressure produced between the photosensitive belt and the developing rollers nor adjust fluctuation of the tensile force applied to the photosensitive belt in the contacting area of the photosensitive belt with the developing rollers, and in the contacting area of the photosensitive belt and the developing rollers, improper images may be produced because the relative speed between the photosensitive belt and the developing rollers still fluctuates or the running speed of the photosensitive belt changes. 
In particular, in developing devices that make the contact development using one-component developers, a sufficient amount of toner is supplied to the nip portion formed between the photosensitive belt and the developing rollers generally by increasing the running speed of the developing rollers higher than the running speed of the photosensitive belt and toner image fog is reduced.  Thus, as the photosensitive belt and the developing rollers run by keeping a prescribed relative speed, if the pressure applied to the nip portion changes and a large pressure is applied between the photosensitive belt and the developing rollers during the running, the relative speed of the photosensitive belt and the developing rollers changes and developing characteristic changes, and improper images many be produced for improper development. 
Further, in recent years, in a full color image forming apparatus, it is demanded to make developing devices and developing rollers small in size for making the entire apparatus small in size and light in weight.  Therefore, when the developing rollers is made in a small diameter, in order to get a good image density in a sufficient developing time by performing the developing operation stably, it becomes necessary to secure a contacting area of the photosensitive belt and the developing rollers sufficiently by bringing them in contact with each other so as the developing rollers are intruded into the photosensitive belt.  When the developing rollers are intruded into the photosensitive belt so as to intrude into there, even if the developing rollers and the photosensitive belt are not sufficiently paralleled, the developing rollers and the photosensitive belt are almost uniformly contacted each other for the overall length of the main scanning direction (the right angle direction to the running direction) and a uniform image density is obtained. 
However, if the developing roller is excessively intruded into the photosensitive belt, the tensile force applied to the photosensitive belt changes largely when the photosensitive belt is in contact with the developing roller and when separated, and the running speed of the photosensitive belt may change largely by the change in torque applied to the driving device of the photosensitive belt and as a result, such improper images as elongation or contraction of images or color deviation of toner images in plural colors may be caused. 
Accordingly, in order to prevent the deterioration of image quality described above, in an image forming apparatus to get a toner image by bringing the photosensitive belt and the developing rollers in contact with each other or separating them, it has been desired to get an image of good quality by obtaining a stabilized and sufficient developing density without impairing the small sized apparatus, adjusting the pressure applied to the contacting area of the photosensitive belt and the developing rollers, and adjusting the change in the tensile force applied to the photosensitive belt when the photosensitive belt and the developing rollers are in contact with each other and when they are separated. 
It is an object of the present to provide a small size image forming apparatus without deteriorating quality of images. 
It is another object of the present invention to provide an image forming apparatus which forms a good toner image by preventing a large pressure applied to a contact area of a photosensitive belt and developing rollers when developing an image by bringing the photosensitive belt in contact with or separating from the developing rollers and a relative speed of the photosensitive belt and the developing rollers from changing, and stabilizing developing characteristic. 
It is a further object of the present invention to provide an image forming apparatus for forming a good toner image by preventing fluctuation of the tensile force applied to the photosensitive belt when the photosensitive belt is brought contact with the developing rollers irrespective of the developing rollers made small. 
According to the embodiments of the present invention, an image forming apparatus comprising: an endlessly running image carrier; a latent image forming portion for forming a latent image on the image carrier; developing devices having developing members that are capable of contacting/separating to/from the image carrier after passing the latent image forming portion and forming developing image by supplying developers to the latent image; and supporting members contacting the surface of the image carrier opposite to the surface contacting the developing devices when the developing members contact the image carrier and supporting the image carrier movably according to the contacting force of the developing member with the image carrier is provided. 
Further, according to the present invention, there is provided an image forming apparatus comprising: a belt-shaped photo-conductor; a latent image forming portion for forming a latent image on the photo-conductor; developing devices having developing rollers that are capable of contacting/separating to/from the photo-conductor after passing the latent image forming portion and for forming developing images by supplying developers to the latent image; developing device moving members for moving the developing devices and bringing the developing rollers in contact with or separating from the photo-conductor; and supporting members contacting the surface of the photo-conductor opposite to the surface contacting the developing rollers of the photo-conductor when the developing rollers contact the photo-conductor and support the photo-conductor movably according to the contacting force of the developing rollers with the photo-conductor is provided. 
Further, according to the present invention, there is provided an image forming apparatus comprising: a belt-shaped photo-conductor; a latent image forming portion for forming a latent image on the photo-conductor; developing devices that are capable of contacting/separating to/from the photo-conductor after passing the latent image forming portion and have developing rollers for forming developing images by supplying developers to the latent image; a photo-conductor moving member for moving the photo-conductor to contact or separate the developing rollers; and supporting members that contact the surface of the photo-conductor opposite to the surface contacting the developing rollers when contacting the developing rollers to the photo-conductor and support the photo-conductor movably according to the contacting force of the developing rollers with the photo-conductor.