This paper deals with nonstationary rotor vibration due to collision with an annular guard during passage through a critical speed. An unbalanced Jeffcot rotor is accelerated at a constant angular acceleration and strikes the annular guard supported by a spring and damper. This dynamic process is calculated by the Runge-Kutta method under various conditions, and the effects of several parameters on the process are discussed. It is assumed in the calculation that a contact force and a friction force are exerted during collision. The contact force or the impulse of the contact force is estimated by the following two methods. (1) By using both the law of conservation of momentum and the coefficient of restitution, the impulse of the contact force is obtained as a momentum change before and after collision. (2) The contact force is assumed to be proportional to an overlapping displacement between the rotor and the guard.
