Abstract: A dynamics model of a rod-shaped rotationalexpansion mechanism with consideration of the flexibility of the centerrod is developed forstudying the dynamic property in the expanding process. A numericalsimulation is carried out under an ideal operating condition. The kinematicparameter in the computed results agree well with the test result. Finally asimulation is carried out with considerations of operationdirection, dynamic deflection and mass unbalance. It is shown that the influence of operation direction and mass unbalanceis much more important, and that of the dynamic deflection could beignored in the small angle area.