Reduction gear unit and image forming apparatus incorporating same

A reduction gear unit, comprising:

• a casing;
• and
• a plurality of planetary gear mechanisms arranged in series in an axial direction thereof within the casing, each of the planetary gear mechanisms including:
• a sun gear;
• an outer gear arranged coaxially with the sun gear;
• a plurality of planetary gears arranged between the outer gear and the sun gear at equal intervals in a circumferential direction thereof and meshing with the sun gear and the outer gear;
• and
• a carrier rotatable coaxially with the sun gear and the outer gear to rotatably support the planetary gears,
• an extreme upstream planetary gear mechanism of the plurality of planetary gear mechanisms in a drive transmission direction including an input element connected to a drive source,
• an extreme downstream planetary gear mechanism of the plurality of planetary gear mechanisms in the drive transmission direction including an output element connected to a member to output the drive force to a rotary member,
• any planetary gear mechanism of the plurality of planetary gear mechanisms other than the extreme downstream planetary gear mechanism including an output element connected to an input element of an adjacent planetary gear mechanism located downstream of any planetary gear mechanism in the drive transmission direction,
• the sun gear of the extreme downstream planetary gear mechanism in the drive transmission direction having a larger pitch diameter than the sun gear of any other planetary gear mechanism, and
• the output element of the extreme downstream planetary gear mechanism is fixed to the carrier of the extreme downstream planetary gear mechanism, and the output element is rotatably unsupported by an axial end cap of the casing so that the carrier of the extreme downstream planetary gear mechanism is floatingly supported in the casing.