Currently, researches on planar gears (including cylindrical gear, noncircular gear) are primarily concerned with those of fixed center distance, that is, the center distance between two meshing gears remains unchanged during the driving process, while those of variable center distance are rarely touched upon. Although not widely applied as fixed center distance gear is, variable center distance gear can be found in noncircular gear shaping and high-torque hydraulic motor, exhibiting unique mechanical properties.