US Patent 7362993 Transfer device and image forming apparatus

In a transfer device, a driving roller that rotationally drives a belt member satisfies a condition “(R2−R1)π×(s−1)×p/(R1×π×2)≦0.15 millimeter”, where R1 is a diameter of the driving roller under an environment of a minimum temperature and a minimum moisture in a use environment range designed, R2 is a diameter of the driving roller under an environment of a maximum temperature and a maximum moisture in the use environment range at design, p is an arrangement pitch of the transfer units, s is a total number of the transfer units, and π is the circle ratio pi.