1. Field of the Invention
The present invention relates to an image forming apparatus or the like which is used in a printer, a copier, a facsimile or the like, and more particularly to an image forming apparatus or the like in which plural image carriers are arranged in parallel.
2. Description of the Related Art
As an image forming apparatus using an electrophotographic method, a so-called tandem-type image forming apparatus in which plural image forming units of yellow, magenta, cyan and black are arranged in parallel is popularly used. In the tandem-type image forming apparatus, in general, a photosensitive drum having a photosensitive layer on an outer peripheral surface thereof is provided to each image forming unit. At a position which faces the photosensitive drums in an opposed manner, a transfer belt which is extended over plural expansion rolls is arranged. Toner images of respective colors which are formed over the respective photosensitive drums are sequentially transferred onto the transfer belt directly or by way of a recording sheet (various types of sheets including a sheet, an OHP sheet or the like) in a multiplied manner. Further, there also exists an image forming apparatus in which a transfer roll which is brought into contact with each photosensitive drum is provided for every image forming unit and a transfer action of a recording material is performed by the photosensitive drum and the transfer roll.
In this tandem-type image forming apparatus, it is necessary to transfer respective color-component images formed by the respective image forming units to a same image carrier position of the transfer belt or a same recording material in a sequentially overlapped manner and hence, there arises a drawback that the registration of the respective color-component images is difficult. Accordingly, with respect to techniques described in publications on the related art, there exists a technique in which a single drive motor is used as a drive system and plural photosensitive drums which are arranged in parallel are driven by the single drive motor. To be more specific, there exists the technique in which plural worm gears are mounted on a rotary drive shaft which is connected to and rotatably driven by the drive motor, worm wheels are mounted on rotary shafts of the respective photosensitive drums, and the worm gears and the worm wheels are meshed with each other whereby the plural photosensitive drums are driven by the single drive motor (see, for example, JP-A-63-11967 (pages 3 to 4, FIG. 1), also referred to as “patent literature 1” hereinafter).
According to the technique described in the patent literature 1, compared to a case in which the respective photosensitive drums have drive sources individually, the registration of the respective color-component images is facilitated. However, mainly attributed to meshing having a backlash between the worm gear and the worm wheel, there has been found a technical drawback that the speeds of the photosensitive drums vary. In view of the above, there has been proposed, for example, a technique which provides a centering position constraining device for rotatably holding one ends of the worm shafts while centering one ends of the worm shafts and constraining both end positions of each worm shaft with respect to the axial direction whereby the image irregularities attributed to the speed variation of the photosensitive drums can be suppressed (see, for example, JP-A-2000-131918 (pages 4 to 5, FIG. 1), also referred to as “patent literature 2” hereinafter).
In this manner, by adopting the technique described in JP-A-2000-131918, it is possible to suppress the speed variation attributed to the oscillation variation and the axial oscillation of the worm shafts. However, the related art described in the patent literature 2 is not provided to suppress the speed variation caused by geometric factors such as the tooth shape accuracy, the pitch accuracy and the like of the worm gear as a single body. To maintain a certain rotation with high accuracy by suppressing the speed variation attributed to the geometric factors such as the tooth shape accuracy, the pitch accuracy or the like, it is necessary to manufacture the worm gears per se with high accuracy.
In manufacturing the general worm gear used in the tandem-type image forming apparatus, a metal material such as stainless steel is formed into a worm shape by cutting machining, rolling forming or the like and, thereafter, for ensuring the accuracy of the gear, a gear grinding step for grinding teeth of the gear is provided separately. Accordingly, to obtain the tooth shape having high accuracy, the manufacturing cost is eventually pushed up whereby there arises a drawback that a product cost is extremely pushed up.