1.  Field of the Invention
The present invention relates to an image forming device such as a facsimile machine, a copier, or a printer or the like (including a Multi Function Peripheral of the facsimile machine, the copier, and/or the printer).  More specifically, the present invention relates to a power transmission mechanism of a drum unit in the image forming device.  The drum unit, which defines an electrophotographic printing unit, can be removably inserted into a device main body from a front side of the device main body along a longitudinal direction thereof. 
2.  Description of the Related Art
An image forming device in which a drum unit can be inserted from a front side of a device main body along a longitudinal direction thereof is widely used in terms of handling convenience, etc.  In such an image forming device, a mechanism for transmitting power to the drum unit is provided on a rear side of the device main body due to restrictions in the configuration of the device.  Accordingly, a drive motor and a drive transmitting gear are attached to a frame on the rear side.  In particular, a gear for transmitting power from the motor to a photoconductive drum of the drum unit is supported by a stud such that the gear can axially rotate.  The stud is attached to the frame on the rear side of the device main body.  When the drum unit is inserted into the device main body, a driven transmission gear, which is provided on the photoconductive drum, and the drive transmitting gear are engaged with each other.  In such a case, an end portion of the stud is either unsupported (in a so-called cantilever state), or supported on a plastic or resin (hereinafter plastic) gear case, which is fixed to the frame. 
Generally, when a process unit and a drum unit are provided in a device main body, and driving force is transmitted from the device main body to the process unit and the drum unit, a load torque is generated in each process mechanism unit (such as a photoconductive drum and a developing roller), which defines the process unit and the drum unit.  Consequently, displacement may easily occur between the process unit and the drum unit.  When such displacement occurs, the image quality is affected.  Therefore, it has been conventionally suggested that an amount of displacement of the developing roller be preassumed so that the developing roller can be placed in the correct position when driven. 
In the above-described image forming device, since the stud that supports the gear is either in a cantilever state or just supported on the plastic gear case at the end portion of the stud, an end of the stud may be swung by the load torque of the photoconductive drum when driven.  Since such a swing occurs in a direction apart from a center axis of the photoconductive drum, a blur may occur in the image to be formed.  In particular, in order to stabilize the drive transmission to the photoconductive drum, the drive transmission position is preferably set as close as possible to a center of the photoconductive drum in a longitudinal direction thereof.  In such a case, however, the stud increases in length, the swing becomes larger, and consequently, the blur occurs more frequently.