The present application claims priority to Japanese Patent Application No.  2001-378488 filed Dec. 12, 2001, the entire content of which is hereby incorporated by reference. 
1.  Field of the Invention
The present invention relates to a tandem-style color image forming apparatus. 
2.  Description of the Related Art
A tandem-style color image forming apparatus 50 like the one shown in FIG.  5 is known in the conventional art.  This image forming apparatus 50 includes an intermediate transfer belt 56 that is suspended over a driving roller 52 and a driven roller 54 and is driven to rotate in the direction of the arrow A.
Four image forming units 60Y, 60M, 60C and 60K that respectively correspond to yellow (Y), magenta (M), cyan (C) and black (K) are aligned under the intermediate transfer belt 56.  The image forming units 60Y, 60M, 60C and 60K each have a cylindrical photoconductor 62 that is in contact with or close to the intermediate transfer belt 56.  They also each have a developing device (not shown) that develops the electrostatic latent image formed on the surface of the photoconductor 62 into a toner image using yellow toner, magenta toner, cyan toner or black toner. 
An optical system 64 is disposed under the image forming units 60Y, 60M, 60C and 60K. The optical system 64 comprises a laser exposure device 66, multiple mirrors 68, etc.  Each photoconductor 62, which has been uniformly charged, is exposed via laser by this optical system 64, and consequently an electrostatic latent image is formed on the surface of each photoconductor 62. 
The electrostatic latent image formed on the surface of each photoconductor 62 is developed by each developing device into a toner image.  The toner images of each color respectively formed on each photoconductor 62 then undergo primary transfer in which they are sequentially overlapped onto the intermediate transfer belt 56.  The four color toner images transferred onto the intermediate transfer belt 56 are conveyed to the nip area between the intermediate transfer belt 56 and the transfer roller 58, and are then transferred together at a nip area onto a sheet of paper that is being conveyed in the direction of the arrow B, based on the operation of the transfer roller 58. 
In the image forming apparatus 50 having this construction, a low-frequency speed fluctuation occurs in the intermediate transfer belt 56 due to vibration that occurs when the driving roller 52 rotates.  If the toner images of each color formed by the image forming units 60Y, 60M, 60C and 60K are transferred to the intermediate transfer belt 56 without taking into consideration this speed fluctuation, the positions at which the toner images of each color are transferred become misaligned from each other, resulting in a color shift. 
In order to prevent color shift attributable to speed fluctuation in the intermediate transfer belt 56, in the tandem-style color image forming apparatus 50, a method that sets the distances between the photoconductors 62 of the image forming units 60Y, 60M, 60C and 60K to a value equal to an integer multiple of the outer circumference length L of the driving roller 52 (1xc3x97, for example) is adopted in general.  It is known that color shift among the four toner images of the different colors due to the low-frequency speed fluctuation of the intermediate transfer belt 56 can be prevented using this method. 
However, where the photoconductors 62 are to be disposed at equal intervals that are each equal to an integer multiple of the outer circumference length of the driving roller 52 between each photoconductor 62, the degree of freedom in arranging the image forming units 60Y, 60M, 60C and 60K becomes restricted, and a dead space 70 is created within the image forming apparatus 50 due to the relationship between the size of the optical system 64 and the space in which the various components are disposed.  As a result, the interior space of the apparatus cannot be efficiently used, hindering efforts to reduce the size of the apparatus. 
An object of the present invention is to increase the degree of freedom in the arrangement of the multiple image forming units that each include a photoconductor in comparison with when all of the photoconductors are disposed at equal intervals, while preventing color shift among the developed images of each color on the transfer belt or the like.  Another object is to enable efficient use of the interior space of the apparatus in order to enable the apparatus to be reduced in size. 
In order to attain these objects, the image forming apparatus of the present invention is an image forming apparatus in which multiple photoconductors that respectively correspond to developers of different colors are disposed along a transfer belt or paper conveyance belt, wherein the photoconductors are disposed at intervals that are each an integer multiple of the outer circumference length of the driving roller that drives the transfer belt or paper conveyance belt, and wherein the distance between at least one pair of photoconductors is made different from the distances between the other pairs of photoconductors. 
The colors of the developers (toner) may be yellow, magenta, cyan and black, and the distance between the black photoconductor and the photoconductor adjacent thereto may be made larger than the distances between the yellow, magenta and cyan photoconductors. 
The photoconductors may be disposed such that they face each other across the transfer belt or paper conveyance belt. 
The colors of the developers may be yellow, magenta, cyan and black, and the image forming unit including the black photoconductor may be made larger than the image forming units including the photoconductors for the other colors. 
The distance between at least one pair of photoconductors may be made larger than the distances between the other pairs of photoconductors. 
The distance between at least one pair of photoconductors may be made twice as large as the distances between the other pairs of photoconductors. 
The distances between each of the other pairs of photoconductors may be made equal to the outer circumference length of the driving roller that drives the transfer belt or paper conveyance belt. 
The image forming apparatus may include an optical system that writes images onto the photoconductors, and a part of the optical system may be disposed in the space comprising the distance between photoconductors that is larger than the distances between other photoconductors. 
Furthermore, the image forming apparatus of the present invention is an image forming apparatus in which multiple photoconductors that respectively correspond to developers of different colors are disposed along a transfer belt or paper conveyance belt, wherein such image forming apparatus has an optical system that writes images onto the photoconductors, and wherein the distance between at least one pair of photoconductors is made larger than the distances between the other pairs of photoconductors and a part of the optical system is disposed in the space comprising the larger distance. 
The photoconductors may be separated from each other by a distance comprising an integral multiple of the outer circumference length of the driving roller that drives the transfer belt or paper conveyance belt. 
Furthermore, the image forming apparatus of the present invention is an image forming apparatus in which first through fourth photoconductors that respectively correspond to developers of different colors are disposed along a transfer belt or paper conveyance belt, wherein the first and second photoconductors are separated from each other by a first distance, the third and fourth photoconductors are separated from each other by the first distance, and the second and third photoconductors are separated from each other by a second distance that is different from the first distance. 
The first distance and the second distance may each be an integer multiple of the outer circumference length of the driving roller that drives the transfer belt or paper conveyance belt. 
The second distance may be twice as large as the first distance. 
The image forming apparatus may include an optical system that writes images onto the first through fourth photoconductors, and a part of the optical system may be disposed in the space between the image forming unit including the second photoconductor and the image forming unit including the third photoconductor. 
The photoconductors may be disposed such that the first and second photoconductors and the third and fourth photoconductors face each other across the transfer belt or paper conveyance belt. 
The image forming apparatus of the present invention is an image forming apparatus in which first through fourth photoconductors that respectively correspond to developers of different colors are disposed along a transfer belt or paper conveyance belt, wherein the first and second photoconductors are separated from each other by a first distance, the second and third photoconductors are separated from each other by the first distance, and the third and fourth photoconductors are separated from each other by a second distance that is different from the first distance. 
The first distance and the second distance may each be an integer multiple of the outer circumference length of the driving roller that drives the transfer belt or paper conveyance belt. 
The second distance may be twice as large as the first distance. 
The colors of the developers may be yellow, magenta, cyan and black, and the fourth photoconductor may be the black photoconductor. 
The image forming unit that includes the black photoconductor may be made larger than the image forming units that respectively include the photoconductors for the other colors. 
The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description taken in conjunction with the accompanying drawings.