In general, an image forming apparatus employs a drive-force transmission mechanism (e.g., gear transmission mechanism) to transmit a driving force from a driver unit to a driven unit via rotating members such as gear.  Such driver unit is fixed in the image forming apparatus, and such driven unit maybe detachable from the image forming apparatus. 
The driver unit includes a drive-force output gear, and the driven unit includes a drive-force input gear, for example. 
When the driven unit is set in a given installation position in the image forming apparatus, the drive-force output gear and drive-force input gear can engage under a given condition. 
When the driven unit is installed in the image forming apparatus, the drive-force input gear of the driven unit contacts the drive-force output gear at first.  Then, either one of the gears rotates to some degree so that the both gears can engage in a proper condition. 
In such image forming apparatus, the driver unit and drive-force output gear may be connected each other by a first drive-force transmission mechanism, and the driven unit and the drive-force input gear may be connected with each other by a second drive-force transmission mechanism, wherein such transmission mechanisms connect components such as gear. 
Accordingly, when installing the driven unit in the image forming apparatus, the driven unit may receive a load stress from the driver unit or the above-mentioned transmission mechanisms because the drive-force input gear and drive-force output gear contact and rotate with each other as above-mentioned. 
In such condition, a user may need to exert a force, which overcomes such load stress when installing the driven unit in the image forming apparatus, which is not a user-friendly phenomenon. 
FIG.  1 shows one related art of drive-force transmission system in an image forming apparatus. 
As shown in FIG.  1, a driven unit includes a drive-force input gear 123, and a drive-force output gear 134 is provided in the image forming apparatus, wherein the drive force input gear 123 can engage the drive-force output gear 134. 
As shown in FIG.  1, the drive-force output gear 134 concentrically engages a drive-force transmission gear 133 with a rotational shaft 135 as a common shaft, which is driven by a driver unit (not shown).  Therefore, the drive-force output gear 134 and drive-force transmission gear 133 are rotatably supported by the rotation shaft 135. 
As shown in FIG.  2, the drive-force output gear 134 includes two first projections 134a, wherein the two first projections 134a extend in an axial direction of the drive 5 force output gear 134 and face the drive-force transmission gear 133. 
The two first projections 134a are provided with a same interval on the circumference of the drive-force output gear 134 as shown in FIG.  2. 
As shown in FIG.  3, the drive-force transmission gear 133 includes two second projections 133b, wherein the two second projections 133b extend in an axial direction of the drive-force transmission gear 133 and face the drive-force output gear 134. 
The second projections 133b are provided with the same interval on the circumference of the drive-force transmission gear 133 as shown in FIG.  3. 
As shown in FIG.  1, the first projections 134a and second projections 133b may contact each other when the drive-force transmission gear 133 rotates around the rotational shaft 135. 
Accordingly, when the drive-force transmission gear 133 rotates by a driving force from the driver unit, the second projections 133b contact the first projections 134a of the drive-force output gear 134. 
With maintaining a contact condition of the first projections 134a and second projections 133b, the drive-force output gear 134 rotates with the drive-force transmission gear 133, wherein the drive-force output gear 134 and drive-force transmission gear 133 have the rotational shaft 135 as a common shaft. 
As shown in FIG.  1, the image forming apparatus includes a play between the first projections 134a and second projections 133b.  
When installing the driven unit in the image forming apparatus, the drive-force input gear 123 contacts the drive-force output gear 134 in the image forming apparatus. 
Under this condition, if the drive-force output gear 134 can be rotated within the above-mentioned play, the drive-force output gear 134 may be rotated without receiving a load stress from the drive-force transmission system in the driven unit or driver unit. 
In such image forming apparatus, after installing the driven unit in the image forming apparatus, the driver unit is driven to contact the first projections 134a and second projections 133b, which are in a non-contact condition, before starting an image forming operation in the image forming apparatus. 
After contacting the first projections 134a and second projections 133b, the driver unit is driven again to start the image forming operation in the image forming apparatus. 
However, when the first projections 134a and second projections 133b contact each other, the first projections 134a and second projections 133b may impact each other with some speed, by which a larger load stress may be applied to the driver unit or tooth plane of gears (e.g., 123, 133) instantaneously, and may result in a malfunction of drive-force transmission system of the image forming apparatus. 
In another related art, an image forming apparatus includes a developing unit detachable from the image forming apparatus.  The developing unit includes a drive-force input gear and developing sleeve (as rotating member), which are connected by a common rotation shaft. 
Similarly to the above-mentioned drive-force transmission gear 133 and drive-force output gear 134, the drive-force input gear and the rotational shaft of the developing sleeve include projections to be contacted each other. 
Accordingly, when the drive-force input gear rotates with a driving force by a driver unit, projections on the drive-force input gear contact projections on the rotational shaft of the developing sleeve, by which the developing sleeve can rotate. 
Play is also provided between the projections on the drive-force input gear and projections on the rotational shaft of the developing sleeve. 
When the developing unit is installed in such image forming apparatus, the drive-force input gear of developing unit contacts a drive-force output gear in the image forming apparatus, and the drive-force input gear rotates within the play between the above-mentioned projections. 
Furthermore, such image forming apparatus includes a coil spring between the rotation shaft of the developing sleeve and drive-force input gear. 
The coil spring applies a bias force to the rotational shaft of the developing sleeve and drive-force input gear so that the projections can contact each other when the developing sleeve rotates in a first direction. 
Such bias force of the coil spring is smaller than a rotational torque of the developing sleeve, and the drive-force input gear of the developing unit rotates in a second direction, which is opposite to the first direction when installing the developing unit in the image forming apparatus.  A load stress which may be generated by the bias force of the coil spring when installing the developing unit in the image forming apparatus may be smaller than a stress caused by a rotation of the developing sleeve. 
When the developing unit is installed in the image forming apparatus, the above-mentioned projections may be in a non-contact condition. 
Therefore, when the developing unit is activated for the first time after installing the developing unit, the above-mentioned projections may impact each other with a some speed, by which a larger load stress may be applied to a driver unit or tooth plane of gears in the image forming apparatus instantaneously, and may result in a malfunction of drive-force transmission system in the image forming apparatus. 
With the advent of barrier-free trend and universal design, manufactures have been requested to produce machines or tools having improved accessibility for people including disabilities.  For example, the Rehabilitation Act of the United States requires federal agencies to make their electronic and information technology accessible to people with disabilities.  Under such circumstances, it is desirable to manufacture an image forming apparatus having improved accessibility and operability for people including disabilities.  For example, it is preferable that an operator can install a driven unit into the image forming apparatus with less operating force.