| Rotor instability, or rotor nonsynchronous natural frequency vibration, is of major concern in the rotordynamics field. Efforts have been made continuously to explain this phenomenon since it was identified more than seventy years ago. To date, rotor instability vibration has been almost ascribed to two mechanisms, i.e., internal friction effects and fluid actions. It has been predicted by some investigations that dead band clearance in bearing supports can induce only sub and/or super harmonics tracking the synchronous speed and does not cause the nonsynchronous (non-harmonic) natural frequency vibration.; It is shown, however, in this dissertation that dead band clearance in ball bearing supports can induce rotor nonsynchronous natural frequency vibration. The computer simulations show that in a limited speed range, the ball-bearing outer race of an unbalanced rotor may leave (tight-loose) then touch (loose-tight) the housing surface and induce an internal impact. Internal impacts will excite the natural frequency vibration of the rotor, which is independent of the rotor synchronous vibration and will reach a limit cycle due to system's damping. Beyond the speed range, the nonsynchronous natural frequency disappears. Half-frequency vibration is also predicted by the computer simulations. It is shown that the half-frequency vibration is induced by the nonlinear coupling between the equations of motion due to dead band clearance.; Experiments with a rotor supported on ball bearings with dead band clearance are conducted. The experimental results agree well with the theoretical predictions.; The nonsynchronous natural frequency vibration of ball-bearing supported rotors (without seals) has been traditionally explained as internal friction induced rotor instability. The classical internal friction theory is discussed in this dissertation. It is claimed here that what is predicted by the classical linear theory on internal friction is a divergent (Liapunov unstable) response and hence is unmeasurable. But rotor instability vibration, which has two significant components at the synchronous and the natural frequencies, is measurable and hence is Liapunov stable. |
