Pinion Gear Second Critical at 2X

[FRANKOK]

Pinion is in a VFD motor driven double helical gear with upmesh speed increaser driving a compressor. As for many similar designs the first lateral critical speed is above operating speed. However there is a large 2X amplitude peak mainly on the coupling end when speed is at 1/2 the calculated first critical (mode shape with high coupling end amplitude). Coupling is a disk pack. Vibration lowers past the resonant peak at higher speed. The peak occurs in the operating speed range.

Cold alignment has been double-checked and problem occurs on duplicate train.

Any documentation of similar cases or technical papers to prove cause was misalignment?

Besides hot train misalignment has anyone found a different cause?