Nonlinear unbalance responses of vertical flexible rotors supported by oil film damper bearings with centering springs are theoretically investigated. The patterns of unbalance responses depending on the amount of unbalance are shown. There exists a critical unbalance α
crit that is defined as an unbalance above which the amplitude at the mass point becomes infinitely large at the critical speed, and there are two kinds of bistable phenomena, one of which occurs under the unbalance below α
crit and the other of which occurs under the unbalance above α
crit. Moreover, the values of α
crit depending on the design condition of rotor-bearing system are obtained, and the guidline for designing rotor system is given.
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