Rotary drive device and image forming apparatus

A rotary drive device comprising:

• a drive motor;
• a drive transmission portion configured to transmit a rotational drive force of the drive motor to a driven rotor;
• and
• a rotation information detector configured to detect rotation information of the driven rotor,
• a drive signal to be fed to the drive motor being controlled based on the rotation information of the driven rotor obtained by the rotation information detector so as to cause a rotation speed of the drive motor to achieve a target rotation speed,
• the drive transmission portion including three or more drive transmission members,
• a high stiffness member being attached to a low-stiffness drive transmission member, the low-stiffness drive transmission member being one, the one being lower in stiffness, of two drive transmission members forming a drive transmission point corresponding a resonant point of a lowest resonance frequency among frequencies at a plurality of resonance points in frequency response characteristics derived from the rotation information of the drive transmission members, to increase stiffness of the low-stiffness drive transmission member,
• one drive transmission point of the low-stiffness drive transmission member, the one drive transmission point being on any one of a drive-transmission input side and a drive-transmission output side, other than the drive transmission point corresponding to the resonant point of the lowest resonance frequency being configured to transmit the rotational drive force through the high stiffness member.