Abstract: The possibility and the regularity of occurrence of nonsynchronous orbits ard investigatedbased on experiments of a Jeffcott rotor-squeeze film damper(SFD) with/without a centralizingspring system. The influences of system parameters,such as radial clearance ratio, load length,unbalance and centralizing spring stiffness,on nonsynchronous orbits and the relationbetween nonsynchronous orbits and rotor vibration are studied. It is found that the centralizing spring is a very effective means to suppress the nonsynchronous ordit.The unbalanceof the rotor is a main cause for occurrence of the nonsynchronous orbits. 1/2 subsyn-chronous orbit can be observed no matter whether the bearing parameter is quite small or very large provided the unbalance of the rotor is large enough. In some cases,the subsynchronous orbits with many inter loops can be obtained. Analytical and experimental results are found in good agreement.