This thesis addresses the experimental investigation of rotor-retainer-bearing-contacts due to a failure of active magnetic bearings. The thesis gives attention to the dynamics of the rotor system which contains roller bearings as retainer bearings. The work focuses on the development of the most critical state that can occur, the backward whirl motion, which is initiated by friction forces acting on the rotor. By employing a high-speed video analysis and image recognition techniques, the positions of the rotor, the inner ring, and the rolling elements of the retainer bearings during the contact procedure of the drop-down are detected. Studies presented in this thesis lead to two kinds of information regarding the understanding of rotor-retainer-bearing-interaction. Firstly, the found speeds and accelerations of the inner ring and the rolling elements shed light on the run-up and run-out behavior of the retainer bearing interacting with the rotor. Secondly, the friction forces, which can not be measured directly, are calculated from the accelerations of the bearing elements using a model-based approach. In a second set of tests, the initial and boundary conditions which lead to the whirling of the rotor are analyzed experimentally. The roller bearings in various stages of wear are examined in a combination with and without the use of additional damper-elements. These analyses are used to generate stability maps that show the influences of rotor position, rotational frequency and kind of retainer bearing unit on the system behavior following a rotor crash. Employing the information generated in this thesis, a new type of retainer bearing construction is developed and analyzed. The new type of retainer bearing prevents the rotor from forming a whirl motion using a non-circular air gap. In order to reach a realistic simulation model of rotor-retainer-bearing-contacts, recommendations are given concerning the choice of influence parameters.