The "shorted turns" of a generator field caused uneven heating of the rotor, which caused the rotor to bow and have high unbalance. The bow in the rotor caused very high amplitude synchronous (at running speed) rotor vibrations. These high unbalance forces effectively unloaded the bearings. Because the bearings were cylindrical bore, when unloaded, they would develop sub-synchronous (near first critical speed near 800 rpm) vibrations. The vibrations at mid-span of this 65 ton rotor were calculated to be on the order of 0.140 inches peak to peak.
TRI performed a computer simulation of the entire rotor-bearing system using TRI's proprietary time-transient non-linear model and duplicated the vibration pattern. TRI then changed the bearing design for both bearings to elliptical with some non-standard features, and the sub-synchronous vibrations were suppressed and the synchronous vibrations were limited.
Tilting pad bearings were also simulated and these had even better performance characteristics. Because the elliptical bearings appeared to be sufficient and could be made more quickly, they were chosen.
The results were quite satisfactory.