Figure 1 - One Possible FFT Resulting From A Rub. Unpredictable Plot Due To 
Wave Shape; Time Domain Plots Essential)

Figure 2 - Rotor Is Striking Something
(i.e. The Housing) In The Vertical Direction.
Note The Signal Shape. How Will The FFT 
Treat This Signal Shape ?
Rub Symptoms:
  • Time domain the easiest way to diagnose - a 'truncated' signal is produced (see animation). 
  • Readings should be taken at several radial positions (would you see the above signal shape with horizontal readings ?).
  • FFT can produce numerous harmonics of rpm (like bearing looseness) but also sub-harmonics at 1/2 x RPM in severe cases due to the wave shape (unpredictable results).
  • With a short enough time period collected (~2 shaft rotations), the length of the rub can be estimated.
  • Highest amplitude harmonic may be one that is the closest to one of the component's resonance.