Experiments on the Stability of an Overhung Rotor in Pressure-Dam and Multilobe Bearings

Four sets of bearings were tested on a three-mass overhung flexible rotor: axial-groove, pressure-dam, and two sets of preloaded (0.75 and 0.40) multilobe bearings. The first and second critical speeds were observed at 3200 and 6800 rpm; predicted values were 2960 and 7420, respectively. The instability thresholds were observed to depend strongly on the load orientations for the axial-groove and multilobe bearing. The maximum instability thresholds (optimum load direction for the axial-groove, multilobe (m = 0.75), and multilobe (m = 0.40) bearings were 6700 rpm, 9200 rpm, and 6800 rpm, respectively. For the pressure-dam bearings, it was 6400 rpm and was predicted to be 5800 rpm. The lowest instability threshold for the axial-groove and multilobe bearings were 5750 rpm, 7600 rpm, and 6200 rpm, respectively. When known unbalances were added to the rotor in the pressure-dam bearings, the stability threshold was increased to above the safe running range of the rotor and a brief instability was observed as the rotor traversed the second critical speed. Temporary low level instabilities were also observed for the axial-groove and multilobe (m = 0.40) bearing as the rotor accelerated through approximately twice the first critical speed range.